ih Ha nites i if ALBERT R. MANN LIBRARY New YorK STATE COLLEGES OF AGRICULTURE AND HOME ECONOMICS AT CORNELL UNIVERSITY QK 918.M3 ‘ sitism in plants, PU PLATE A. Large, densely branched plant of Krameria fastened to the younger roots of a slender young plant of Covillea, B., B. THE CONDITIONS OF PARASITISM IN PLANTS BY D. T. MACDOUGAL AND W. A. CANNON WASHINGTON, D. C. PUBLISHED BY THE CARNEGIE INSTITUTION OF WASHINGTON 1910 320189 CARNEGIE INSTITUTION OF WASHINGTON PUBLICATION No. 129 Copies of this Book were first issuer’ SEF 3 1918 THE CORNMAN PRINTING Co. CARLISLE, PA, CONTENTS. Dependent nutrition in seed-plants ............cessseeeeeeessesenseeeeeee The root habits and parasitism of Krameria canescens .... Parasitic seed-plants of the desert.............cccccecccccseeeeecceeeeeceeeaeeeseeaenesecaeeenes Krameria canescens and some features of the Kramerias ..............0ssceeseseeerss Root-system of Krameria canescens Anatomy of the roots The haustorium EnG@hta) Pari OSA s. asnsewvevenesyeceaseosniedsinn vschuaboagih wow soa tateidace seasausalce wowed wenate Ephedra antisyphilitica Franseria Quinoa) svcsisexssssussceavsleusensyecenws neesaanede cenvacanuseceeeeeeueies LyCiumi And €rSOnil sac csesiics sip vsata say sanecedasweaaaeasaaceheaataabasangacanbeocaseanesscatsaek Meéenod oF a SCA bia ieccsusaseeieadas psagebe.o.ockndaa ctor peeven ented REA onsen s Parkinsonia: microphylla sisescesesesescins snivenen. sess seaupevenceeneen wcuevieeveeuannss » Prosopis velutina.............. Zizyphus parryi............... Cultures of Krameria canescens Summarization of general features of parasitism of Krameria..................... Xeno-parasitism: the experimental induction of dependent nutrition........... ...... Material and) tWethOd ss. sic3: gaits: 'icsaacs ocesaetenvas boana bance suansasesmedehe wea acecuitarssbniaesy’ Total and inorganic solids Determination of acidity... Cissus=O pi nti at vas veceesueteey: cain oedeanvee ware penceaiacs tetha net ded peed naa leleseee a ces CissS—EehinOCaGts:..ssiscerucsdsesesuchen casadeenaguoatsaderdsdebeatoes sduacncuaheaoeatmucvenutees GSS GS=C ari CRI OB ses aiovercnsseluscus ars sak No ups ne bereaianta ean ee adeatimeadaan eieapeads ema eneds Cissus—F ouquieria ........... Opuntia—Echinocactus Autonomic movements of Opuntia Opuintia=O pantiaivisscisnwedercssesee vaeeeiganii es sunageunet sce eals sven eorenchegeunededuoendssenys Opuntia-Pouquitria. w2cccessenwsessseesss avon eeeeieaeere a aesaeeeeeea Opuntia—Carnegiea .... sees Eviphorbia=O punta. ..c.c.05..ncses teuseeenessoasessa sanuvareaaioacdaesceavecdievawesnedeece Seebiawesvs Fouquieria—Opuntia, Fouquieria~Carnegiea, and Fouquieria—Echinocactus.... Cotyledon=Opunttial evsccrane siccuuies scvusngseisseina te: veewcen dd seed sua vesievodeneineyeaeawhneeasueds Cotyledon—Carnegiea ............... Cotyledon-Echinocactus Agave-Echinocactus ............44 oe Agave-O puntla sieescacdseseseseravezanthateneetenaadnceraMeesatuanccdoes ouetigeueu es asvecdeandiiss AB aVie=C aTNePi Ca cing sai dessdeancis niche deitine veins Sarssoie na sidnbaaidalsve chacichsGlehace savvonqewtneesaets Tradescantia—O puntia ............. Miscellaneous arrangements Résumé of experimental results The origination of parasitism .....0....ccccssscesesesceesseesscceeecescesaecesansaes tasecssseseeenees 15 ot 25 25 25 26 27 31 34 36 36 37 42 42 42 44 45 45 45 45 45 46 47 48 48 48 30 THE CONDITIONS OF PARASITISM IN PLANTS. DEPENDENT NUTRITION IN SEED-PLANTS. The number of seed-plants which obtain water and dissolved substances from the bodies of other living organisms is large, although the species exhibiting dependent nutrition are confined to a few families. In any nutritive couple, whether in parasitic arrangement or in mycorrhizal sym- biosis, the plant deriving greatest advantage from the association usually ' displays a more or less marked lack of differentiation of the tissues, which” may extend even to the seed and embryonic structures and is also charac- terized by a lessened production of chlorophyll. The capacity to construct this important material has been entirely lost by some forms. The easily made assumption that these morphogenic effects are due to the availability of organic food-material is not borne out by experimental cultures of plants in which substances of this kind were furnished in ample proportions. The effects in question seem to be produced only when two organisms assume a direct physical contact by which the ready passage of material from the body of one to the other is made possible, and this generalization also appears to be capable of strict application to animals. The studies of the general anatomical features presented by the families, the members of which exhibit parasitism, have so far failed to yield any conclusions as to the morphological features which might be favorable to such arrangements. The specialization of tissue which ensues when a seed-plant becomes parasitic fortuitously is far more striking than any simple anatomical character which might be interpreted to indicate a pre- disposition to the dependent habit of nutrition in autophytes. Theoretical considerations lead to the belief that it is to purely physio- logical features and to the habits of green plants that we must look for the conditions favorable to the organization of parasitism. Evidence to the effect that a number of green plants may take in organic compounds through their membranes is increasing, and this capacity would facilitate parasitism as at present understood. The course of development of the absorbing organs, their mechanical relations to the bodies of other plants, and the concentration of the fluids in the cells of the possible parasite would be features to which attention would naturally be directed in any attempt to ascertain the method of origin of this method of nutrition. Some results of importance with respect to this matter are presented in the following pages. 1 2 CONDITIONS OF PARASITISM IN PLANTS. The mechanical adhesion of the bodies of seed-plants which would make parasitism possible might be brought about in three different ways: (i) Roots growing thickly interlaced in the soil might unite or penetrate each other; (2) adventitious roots arising from internodes at any place on the. aerial stems might pierce the bodies of other plants; (3) seeds lodging in the bark or in the wounds of a plant might germinate and send absorb- ing organs into the tissues of the possible host. Of these methods that of incidental root-parasitism seems to bear the greater probability of occurrence and to be illustrated by some very striking examples. Peé-Laby has recently described a case in which the main root of a plant of Passifora cwrulea had become attached to a root of Euonymus japonicus, forming a specialized absorptive tissue and under- going general atrophy of its own root-system, in a manner suggestive of a highly developed degree of parasitism, although, of course, no hint of this was yet to be seen in the shoot of the Passifora. (Peé-Laby, M. E. La Passiflore parasite sur les racines du fusain, Rev. Gen. d. Bot., xvI, 453, 1904.) Similarly Cannon has discovered that A’vameria canescens and K. parvi- folia, desert plants hitherto taken to be autophytic, may fasten upon the roots of a dozen species near which they grow habitually, the structures of the roots being indicative of a stage of modification not far advanced toward complete parasitism. > in a horizontal position, directed away from the window, the total sweep occupying about 47 or 48 hours, perhaps less. The reverse which would carry the tip toward the light began immediately, the tip having risen 1 cm, by 3" 20" p. m., and reached the horizontal position with tip toward the window at 2 p. m. on the 20th. The complete cycle was thus completed in something over 60 hours, the movement away from the window consuming twice as much time as that toward the window. On the next cycle the minor movement at a position near the midway was not noted accurately, but it was not unduly prolonged, for the extreme position with the tip away from the window was reached in 2 days as before. At 7 a.m. on the 23d the tip had retracted 15 mm. from this position, becoming erect by noon, and was found in a horizontal position at 10 o’clock the next morning. The next movement was not observed, but on May 29 it was again noted at the median position, and the minor movement was prolonged so that it had departed from this 4 cm. on the 30th and was found in the extreme position with tip away from the window on the 31st. At 2 p.m. the movement toward the window had raised the tip 3 cm., and this phase of the nutation was completed within the day. (Plate 8.) The movement was now carried out with greater irregularity, which may be ascribed to the rising temperatures. Although the extreme position was reached occasionally, yet the shoot would be held in the median posi- tion for several days, in which it made movements of small amplitude. On July 30 it was noted that no movement of any consequence was to be seen during this and the day previous. On October 5, 1909, the preparation was again brought under observa- tion and it was found that the shoot was moving somewhat irregularly through an arc the chord of which would measure about 9 cm. The tem- peratures were now much lower than midsummer, the shaded laboratory probably not rising above 85° F. On October 15, 1909, the stem was moving through an are the chord of which would measure about 5 or 6 cm., with greater irregularity and shorter cycle than before, the complete movement not occupying more than 2 days. Movements of a second etiolated shoot were observed in May, but were of small amplitude. In October, however, 3 etiolated shoots, in addition to the principal one described above, were found to be in motion, and the tips were seen to change position as much as 3 or 4cm. in cycles of varying length. The analysis of the curvatures by which the movements were produced showed that the entire etiolated shoot was affected. Ifthe movement from the extreme position toward the window be followed, the following pro- cedure would be noted: A concavity on the upper side of the stem would AUTONOMIC MOVEMENTS OF OPUNTIA. 39 be formed which raised only the extreme terminal portion of the stem and which slowly moved down the stem. Meanwhile, the basal portion of the stem was still in the condition of having a concavity on the opposite lower side, so that when the curvature now under description had progressed to the middle of the stem in its downward course, the shoot or branch was in the form of a lax $, with antagonistic curvatures in the apical and basal por- tions. It was in this position that the marked minor movements occurred. Eventually the course of the progressive curvature was continued, and at the end of two days it was sharpest at the base. Now, however, the curvature which would carry the branch toward the window, instead of be- ginning at the tip, would take place on the opposite upper side of the basal portion of the stem and gradually move upward, with the result that by the time it had reached the middle of the stem the tip was brought into the posi- tion directed toward the window. The movement away from the window was by a curvature beginning at the tip and extending downward, 2 days being necessary for it to reach the base of the organ and with antagonisms resulting in minor movements in the median position. The movement toward the window was produced by curvatures beginning at the base of the stem immediately opposed to the curvature, which had just reached that region on the opposite side. Nutations are not easily interpreted, as evidenced by the many laborious attempts made in explanation of them. Positive information does not go beyond the fact that tropistic reactions, particularly to gravity, and the irreversible changes in form due to rhythmic growth-action are involved. Alternation of position of the zone of rapid growth from one side or flank to another is well known, but alternation by which this zone moves up one flank and down the other is quite anomalous. Ink-marks were placed near the base, at the middle, and near the tip of the shoot on the side away from the window, and the two intervals thus laid off were measured from time to time in October. A caliper-scale with vernier dividing the millimeter to tenths was used. The movement now made a cycle which was completed in about 2 days, so that one set of measurements would be in the concavity and the distance between the same points on the fol- lowing day would be obtained by the application of the ruler to a surface now convex. Day | Oe | eee eee || Gua Oct. 9, 1909 Concave. 37.6 28.8 66.4 , Oct. 11,1909 | Convex. 37.5 29.9 67.4 Oct. 12,1909 | Concave. 37.4 29.6 67.0 | Oct. 13,1900 | Concave. 37.8 28.5 66.3 Oct. 14,1909 | Convex. 38.1 29.1 67.2 | Oct. 15, 1909 Concave. 36.8 28.3 65.1 Oct. 16,1909 | Convex. 37.4 . 29.1 66.5 Oct. 17,1909 | Convex. 37.4 29.5 66.9 40 THE EXPERIMENTAL INDUCTION OF DEPENDENT NUTRITION. Preparation now moved near window with southern exposure, as a con- sequence of which it was warmed by the direct rays of the sun. Two additional measurements gave the following results, showing that the total length of the shoot decreased from 67.4 in the earlier part of October to 61.5 in November, a shrinkage of about 7 per cent. ' 7M Date. Convex. | ee | oe Total. | Oct. 24,1909 | Convex. | 37-5; 28.2 66.7 Nov. 6,1909 | Convex. 34.7 | 26.8 61.5 Some of the peculiarities of the nutating movements of etiolated stems have been described by Wiesner, who saw more than one zone of curva- ture in one internode at the sametime. (See Wiesner, J., Die undulirende Nutation d. Internodien, Sitzungsber. d. Wien. Akad., Lxxvur, Abth. 1, 1873, Bewegungsvermogen, p. 22, 1881, and Siszungsber. d. Wien. Akad., Lxxxviul, Abth. 1, p. 454, 1883.) The etiolated shoots of Ofuntia consisted of many internodes, all of which were more than a year old. The transverse section showed a flat- tened oval with a central mass of parenchymatous tissue, comprising about one-third of the shortest diameter. Inclosing this is a flattened oval of 15 to 20 fibro-vascular bundles, with perhaps a few secondary formations, and external to these was a heavy layer of parenchymatous tissue, which in thickness amounted to about one-third of the diameter and had the posi- tion and some of the features of a cortex. The epidermis was simple and with walls not materially thickened. (See fig. 1.) Shoots taken from the dark-chamber and exposed to open-air conditions until they became green, displayed a reduction of the turgidity of this ‘cortical layer,’’ so that the walls were variously contracted and folded, the central parenchyma being only slightly affected. Greened shoots used as xeno-parasites showed a marked accentuation of this collapse of the external parenchyma, which culminated in the wrinkling and death of the epidermis and of the outer layer suggestive of decortication. This process proceeded slowly for several months, and its course is similar to some of the changes which take place in the basal portions of the stems of many of the opuntias. ““All active nutation curvatures are the result of unequal growth on the two sides of the cell or curving organ’’ (Pfeffer, Physiology of Plants, I, p. 12, 1906), but the alternating curvatures of Opuntia under discussion are seen to be due to causes exactly the reverse of those to which nuta- tory phenomena are ordinarily attributed. A second characteristic con- sists of the fact that the entire shoot displays this movement for several months; and the third characteristic, equally notable, is the alternation of curvatures by which the convexity or concavity passes down one side of the shoot to the base and then passes to the opposite side, in which it moves AUTONOMIC MOVEMENTS OF OPUNTIA. 41 gradually tothe tip. The irregular variations seen in the major movement are to be expected in tissues undergoing the changes finally leading to decor- tication. Similar irregularities are seen in the ordinary type of nutations. Fra. 1.—A, diagram of cross-section of etiolated shoot of Opuntia blakeana, showing number and position of fibro-vascular bundles and volume of parenchymatous tissue. B, cross-section of etiolated shoot which has been exposed to light, with shrinkage of the outer ‘‘cortical” layer. It seems very clear that these autonomic movements are not nutations. They ensue only within a certain range of temperature and are undoubt- edly due directly to the disturbance of the balance of turgidity in mov- able zones on opposite sides of the shoots. The shoots were held rigidly in all positions, which leads to the suggestion that the disturbance of the balance must be attributed to a heightened turgidity on one side, although the possibility is not wholly eliminated that a slight reduction of turgidity on the concave side might be the actual cause, without lessening the rigidity of the stem to a perceptible extent. In fact, nothing but purely speculative reasons might be given to support the conclusion that an in- crease of the osmotic activity of the contents of the cells of one side, or a decrease in the other, was the actuating cause. The method of develop- ment of a rhythm by which the alternation of movement ensued would be the same in both cases. It is to be noted that these movements take place when a plant is brought into intimate relation with an environmental factor represented by the host plant. It is probably connected with the restricted water-supply obtained from the host. No aspect of the phenomenon in question suggests 42 THE EXPERIMENTAL INDUCTION OF DEPENDENT NUTRITION. adaptive variation, and the supposition arises that perhaps many unusual activities displayed by organisms under highly specialized conditions are equally non-adaptive, although resulting directly or indirectly from the action of environic factors. OPUNTIA-OPUNTIA. On February 12 an etiolated shoot of Opuntia which had been cut and allowed to regenerate and green in the sand-bed was inserted in a cavity made for it in a rooted joint of Opuntia discata. This seeming to be in good order, the whole preparation was put in the dark-room on March 2, 1909. Slips of O. Zeptocaulis had been inserted in joints of O. discata early in 1908 and these lived and developed new branches indicative of grafting. OPUNTIA-FOUQUIERIA. A number of slips or single joints which had formed calluses at the base were fastened to stems of Fouguieria in places where the epidermal tissues had been removed early in 1909. It was difficult to bring about good contact between the parasite and host, so that most of the preparations were unsuccessful, although plaster was used to seal and hold the parts in position. On July 30, 1909, one such preparation was alive and turgid, although no growth had been made. On October 8, 1909, this prepara- tion was taken down and it was found that the joint of Ofuntia was dead at the base, having no capacity to absorb from the tissues of the host, which were completely callused. OPUNTIA-CARNEGIEA. The series of preparations of these two plants were begun by the use of freshly-cut slips of Opuntia which were inserted in cavities made in the trunks of Carnegiea distances of 1 to 2 meters from the base and sealed with gelatin. The host exudes a quantity of sap, which quickly turns brown and blackish, and the seals were soon destroyed. Later plaster was substituted for the gelatin, and also the practice was followed of making the cavity of such size that the insertion of the slip would close it com- pletely, a thing easily done by reason of the great turgescence of the tissues of the enforced host. The first preparations were made on January 30, 1908. March +, 1908.—The exposed surfaces of the hosts were covered with a hard black coat in which the roots and basal portions of the inserted slips were incased. Of course some moisture might pass through this layer, and the xeno-parasites were all plump and turgid, and were in the same condition as late as May 22, 1908. A number of cylindrical and flat opuntias had been used in the arrangements. A regenerated plant consisting of a basal fragmentary joint of Opuntia blakeana, with roots issuing from the lower surface, was seated snugly on the concave surface of a living stump of Carnegiea which was fully healed and covered with corky plates. The loose plates were pried away. A few weeks later the Opuntia was fixed in place by means of plaster. OPUNTIA-CARNEGIEA. 43 August, 1908.—Several insertions of Opuntia lehtocaulis and O. verst- color were set in a trunk of Carnegiea and sealed with plaster. October 30, 1908.—Three additional healed cuttings of Opuntia versicolor were inserted in the trunk of Carnegiea. The O. dlakeana on stump of a host plant had become firmly embedded. January 23, 1909.—A young plantlet of Opuntia versicolor, in its first year, was taken from the soil, the roots trimmed back, the whole basal part of the plant being inserted in a cavity of a Carnegiea near the glass- house. This cavity was made by cutting out the black incrustation from the surface of a former wound. The preparation was sealed with a liberal supply of plaster. March 30, 1909.—One insertion of Opuntia versicolor on host first used was showing an active bud. April 5, 1909.—An etiolated greened stem of Opuntia was sealed into the freshly-cut upper surface of a Carnegiea, in which similar cuttings of Cissus, Agave, and others were put at the same time. June 12, 1909.--The Opuntia blakeana on stump of host had not made any growth. An insertion of the same plant on the surface of a trunk near by had made no growth. The O. versicolor plantlet on tall host had made no growth and was of a yellowish color. Several of the insertions, including more than one species, were in a dying condition, but the etiolated greened O. blakeana on stump with other insertions was alive and growing. July 30, 1909.—The supply of moisture had not changed the condition of the arrangements on this date, with the exception of those made with Opuntia versicolor. All insertions of this plant were plump and turgid and were showing a fair amount of growth. October 2, 1909.—A review of all the preparations showed that all the etiolated shoots of Opuntia inserted in Carnegiea were dead. Two plants of Opuntia blakeana on Carnegiea had survived the summer and were tur- gid, but had made no growth in size or by addition of joints. The plantlet of Opuntia versicolor on tall host-plant near glass-house was apparently in a healthy condition, although slightly yellowish in color, in agreement with the condition of hundreds of normal specimens growing in the soil. Seven insertions in the basal part of another host were alive. Three of these were dead in the basal portion and were deriving but little from the host. The remainder seemed to have contact of living tissue with host. November 9, 1909.—A dissection of the arrangement of Opuntia blake- ana on sahuaro showed that the base of the stem incased in plaster had given rise to a number of roots, which were closely pressed against the thin, hard, indurated corky layer which is formed by the sahuaro when the air is excluded partially. No actual penetration had occurred, although the Opuntia had not desiccated as if lying free, suggesting that some material had been received from the host. January 8, 1910.—Several preparations of Opuntia versicolor on Carnegiea were still alive, but with the xeno-parasite showing the effects of an inade- quate supply of water. Three were especially plump and turgid. On January 10, 1910, all of the arrangements of Opuntia versicolor upon Carnegiea except three were dissected. The host had in all instances formed a corky layer on the surfaces exposed to contact with the xeno-parasite. Despite this fact the insertions of Opuntia had withdrawn sufficient liquid to preserve a fairly plump condition, though not so turgid as branches of 44 THE EXPERIMENTAL INDUCTION OF DEPENDENT NUTRITION. plants normally rooted in the soil. The explanation for this action is to be found in the high ash-content, implying great osmotic activity of the juices of the xeno-parasite. An analysis of material taken from the ter- minal branches of a plant on January 12, 1910, gave an acidity of the sap equivalent to 0.483 gram H,SO, per 100 c.c., the total solids being 12.040 grams and the inorganic material or ash 2.052 grams in this quantity. A second determination was made on February 9, 1910, in which the total solids were found to amount to 10.66 grams per 100 c.c. of sap, the inor- ganic material or ash being 2.10 grams. The ash did not differ materially in the two analyses, and the difference in the amount of total solids obtained may be attributed to the colloidal nature of the material obtained by press- ure. The average of three determinations of the freezing-point indicated an osmotic activity of 11.98 atmospheres at a temperature of 25° C. This is of importance, as it was the condition existent at the time successful preparations were set up. The concentration doubtless increases with aridity of the dry foresummer and then falls with the summer rains and the growth and expansion of the stems. (For osmotic activity of Carnegiea see p. 35.) The probability of Opuntia becoming at least partially parasitic on Carne- giea led to careful observations in the field on all of the expeditions from the Desert Laboratory. A prickly pear consisting of several healthy joints was seen growing from a cavity made by ‘‘carpentarios’’ in the summit of a Carnegiea about 80 miles west of Tucson in November, 1907 (see plate 9). Neither host nor parasite was disturbed. A plant of Opuntia fulgida about 50 cm. high occupies the soil in a dirt-filled cavity in a stump of Carnegiea on the Yuma Mine road, 9 miles west of Tucson, but here the arrange- ment is hardly parasitic. A small Opuntia blakeana issuing from a knot- hole in the base of a stem of Parkinsonia microphylla was found in 1906 and was dissected for examination. The roots were normally developed, occupying a mass of humus and soil that had fallen in through a second cavity on the opposite side of the small trunk. The proportion of humus was much greater than that of soils ordinarily occupied by this plant, but nothing more than mechanical parasitism might be attributed to the union. The Opuntia was preserved, being set in the soil at the Desert Laboratory, and showed a vigorous growth when transplanted. A similar occurrence of Opuntia on Acacia greggii, 18 miles east of Tuc- son, was found by Dr. Cannon in 1909. (Plate 10, 4.) EUPHORBIA-OPUNTIA. Freshly made cuttings of Euphordia (an African species with cylindrical stems), which had been growing in a shelter near the Desert Laboratory, were inserted in stems of Opuntia versicolor and O. discata early in March, 1908. Free cuttings quickly desiccated, but many of these remained fairly normal for 2 months, one opening 3 flower-buds in succession. No roots were formed, however, and probably the unions were of the nature of grafts. No care was taken to shade the preparations, and no opportunity FOUQUIERIA-OPUNTIA, FOUQUIERIA-CARNEGIEA, ETC. 45 was offered for repeating the experiments with regenerated slips. Similar preparations with Cereus remained alive entirely through the arid fore- summer when inserted in the stems of Carnegiea. FOUQUIERIA-OPUNTIA, FOUQUIERIA-CARNEGIEA, AND FOUQUIERIA- ECHINOCACTUS. A large number of cuttings of Aouguieria, both fresh and regenerated, were inserted in stems of Ofuntia, Carnegiea, and Echinocactus at various times, many with and some without seals. A few of these cuttings were alive after 6 months, their contact yielding some water to them, yet in no case was any activity in the way of leaf-development displayed. Decay ensued in the majority of cases. COTYLEDON-OPUNTIA. February 4, 1909.—Two regenerated cuttings of Cotyledon macrantha were inserted in joints of Opuntia castille which remained alive for several weeks by reason of contact with moist tissues, but no growth was made, the leaves withered, and finally the stems died also. The repetition of the test with O. d/akeana gave similar results. COTYLEDON-CARNEGIEA. On May 17, 1909, two regenerated slips of Cotyledon macrantha were set in cavities on the north side of a trunk of Carnegiea and sealed with plaster, one having been previously arranged. June 12, 1909.—All of the preparations were alive, but no growth had been shown by the parasite. October 11, 1909.—-One of the parasites survived. The stem of the slip appeared plump and turgid and the terminal bud healthy, but no forma- tion of leaves had taken place. November 9, 1909.—-Dissection of the remaining preparation showed no root formation by the inserted member, all growth having taken place at the expense of balance carried, except such as might have been taken in through non-active tissues. COTYLEDON-ECHINOCACTUS. On February 4, 1909, two regenerated slips of Cotvledon macrantha which had been fixed in cavities in the body of an Echinocactus in the open, a few days previously, were alive. One of the slips soon perished, but the other persisted until midsummer, when it was killed by the high temperature. No penetration of the host was effected. AGAVE-ECHINOCACTUS. In May, 1909, a small Agave was set in a cavity in the body of an Echino- cactus in the laboratory and sealed with plaster. The leaves elongated dur- ing the summer, and when the plaster was removed, early in October, 1909, the central stem was swollen, a secure anchorage had been obtained, and three buds were found in the axils of the old leaf-bases which were removed. 46 THE EXPERIMENTAL INDUCTION OF DEPENDENT NUTRITION. Two other slips were set in the body of another plant, one of which showed elongation of the leaves and a swollen base, but the other, which was limp and had not attained a good anchorage, was removed and reset in a cavity freshly made. A number of short branching roots had been formed. These were embedded in sterilized cotton wool and put into the cavity, which was then securely sealed with plaster in the customary manner. On November 8, 1909, the preparations noted above were dissected. Numerous roots had been formed by the inserted slips, which had extended to fill the cavities of the host, but no penetration of living tissues had been effected. The bases were well healed and might be capable of forming a new series which would be capable of extension, perhaps into the body of the host. (Plate 5, C.) No further root-formation had taken place when an examination of the remaining preparations was made during the cool winter resting-period on January 20, 1910. Two analyses of the sap of gave were made. The sap obtained by pressure from the bases of the leaves of plants taken up early in Novem- ber, 1909, was found to contain 11.710 grams of solid material in 100 c.c. of sap, of which 1.090 grams was inorganic material or ash, the remainder, 10.620 grams, being principally sugar. This would indicate a high osmotic activity, which was confirmed by a second analysis made January 27, 1910. Material of the same kind showed 14.662 grams of solid material in 100 c.c. of sap, of which 1.096 grams was inorganic, with a carbohydrate content of 13.556 grams; by the use of the freezing-point method the osmotic pressure of the sap was found to be 10.84 atmospheres as an average of three determinations. No analysis was made of the ash, but on the allow- able supposition that its osmotic activity would be practically equal to that of K,CO,, the osmotic activity of the dissolved salts in the sap would be equivalent to about 7 atmospheres, which would indicate that the organic material exerted a pressure of about 3.6 atmospheres. The failure of Agave to establish efficient nutritive connections with the body of Lchinocactus is to be attributed to the fact that the formation of a loose clump of roots gives sufficient exposure to the air to allow the for- mation of a heavy callus by the trial host. AGAVE-OPUNTIA. A small plantlet of 4gave, with roots trimmed close, was set in a cavity in the upper portion of a joint of Opuntia discata in December, 1908, and a second preparation of the same kind was madein January. A third prepa- ration on O. discata in shelter was made in early February. On May 8, 1909, the two preparations made 5 and 6 months before were dissected. In one, 5 small roots had been formed by the Agave, which AGAVE-CARNEGIEA. 47 lay in an open cavity with hardened walls. The roots did not come into contact with the moist tissues of the host. In the second the cavity was not so large, a dozen roots, many of them branching, had been formed, and their apical portions were in contact with the clear greenish tissue of the host, making possible some degree of parasitism. On July 30, ‘1909, the third preparation was in action when examined. AGAVE-CARNEGIEA. On January 21, 1909, a young Agave with leaves 30 to 40 cm. long was trimmed both as to roots and leaves and the base set in a cavity in the north side of a Carnegiea near the laboratory, being securely sealed by the use of a large amount of plaster. The cavity had been originally formed as a result of injury, had healed perfectly, and was lined with a heavy corky tissue, which was cut away to leave the base of the stem of the Agave in contact with the central cylinder of the stem. The preparation remained intact and had shown no action when examined on July 30, 1909, 6 months later. Six other preparations were made in February, 1909, one being set in the freshly-cut surface of a plant which had been decapitated. This prep- aration was shaded and appeared to have made some growth when exam- ined on July 30, 1909. Two of the other preparations were not in very good condition, and two were apparently normal as to the condition of the parasite. On October 11, 1909, the Agave set in the stump of a sahuaro near the laboratory had made a growth of several centimeters in length of the leaves and was apparently normal. The larger plant, set deep in the cavity in the northern side of another sahuaro, had elongated the inner leaves to a length of several centimeters, and also showed a swollen base indicative of accession of water and food-material. None of the other preparations were in very good order, although still surviving. On November 9, 1909, the Agave set in top of stump of sahuaro was dissected. Roots had been formed which ramified between the corky layer and the plaster, but no penetration had been made. The slip was dried and shrunken as if it had received nothing from the host. On February 3, 1910, all of the preparations had been dissected except the one in which an Agave was set in the side of a tall sahuaro. The apices of the leaves had been slightly affected by the freezing temperatures, but the bases remained plump and healthy. No growth is made by this plant except during the period of higher temperatures. The high osmotic activity of the sap of 4 gave acts to prevent its desic- cation under very arid conditions, and undoubtedly the inclosure of the swollen base of the plant in a cavity with plaster facilitates the withdrawal of some liquid from the tissues of the sahuaro, although separated by very refractory or dense coatings. 48 THE EXPERIMENTAL INDUCTION OF DEPENDENT NUTRITION. TRADESCANTIA-OPUNTIA. The regenerated cuttings of Zradescantia which had been received from Prof. W. J. V. Osterhout were on July 15, 1909, inserted in the upper portions of joints of an introduced Opuntia found in the vicinity of Carmel, and which had been brought to the acclimatization laboratory. July 29, 1909.—The inserted cuttings had flagged, much more so than those left lying on a board in a shaded room. September 12, 1909.—The exposed portions of the slips had died back, but two were showing developing buds on the nodes near the bases of the cuttings. October 6, 1909.—Nine slips of Tradescantia with regenerating bases were setin the upper portions of joints of Opuntia dlakeana in the lath shelter at Tucson, Arizona. Two slips were thrown on the ground as a control test. November 2, 1909.—The insertions at Carmel were found to have per- ished. November 9, 1909.--None of the preparations at Tucson were alive. MISCELLANEOUS ARRANGEMENTS. Plantlets and cuttings of Agave, Fouguteria, Salix, Populus, Vitis, and Echinochloa were inserted in the bodies of Opuntia, Carnegiea, and Echino- cactus, with no success. It is probable, however, that regenerated cut- tings of some of these woody plants might succeed in establishing para- sitic relations if protected from the desiccating effect of wind and sun during the earlier stages of the preparation. RESUME OF EXPERIMENTAL RESULTS. The facts obtained from the experimental arrangements described in the foregoing pages show that it is possible to establish regenerated cuttings of a number of species in a dependent nutritive relation with the bodies of enforced host-plants. Arrangements of xeno-parasitism were made which endured for two seasons or more. The xeno-parasite formed roots which penetrated the tissues of the host in some instances, while in other cases absorption took place through the epidermal tissue of the submerged bases of the in- serted slips. The facts at hand do not warrant any conclusion as to the significance of morphological features in the assumption of nutritive rela- tions between two seed-plants. The development displayed by xeno-parasites was, in all instances, less than that of similar shoots autophytically nourished. The atrophy of the shoot characteristics of parasites was thus displayed as an immediate re- sponse to dependent nutrition. In addition to manifestations which might be classed as direct responses, the etiolated shoots of Opuntia exhibited striking autonomic movements, not attributable to inequalities of growth. These movements appear to be caused by a rhythmic inequality of tur- RESUME OF EXPERIMENTAL RESULTS. 49 gidity in the outer parenchymatous tissues of the stems, which undergo changes simulating decortication. The zones of curvatures producing the movements moved up and down the stems from base to apex in a period of about 72 hours. Successful xeno-parasitism is dependent in the first place upon the superior osmotic activity of the parasitic member of the nutritive couple, although not all pairs of plants sustaining such inequality are capable of becoming host and dependent, and other features act as limiting factors of minor importance. The proportion of salts dissolved in the sap of the experimental plants and the osmotic activity as indicated by freezing-point tests undergo wide seasonal variations, as a result of which a xeno-parasite may maintain itself upon a host during the period of greatest turgidity of the latter and be unable to withdraw material from it during the drier season, when the sap of the host is of a relatively greater concentration. (See Drabble and Lake, ‘‘The Osmotic Strength of Cell-sap in Plants growing under Different Conditions,’’ The New Phytologist, vol. rv, 1905, and ‘“‘The Relation between the Osmotic Strength of Cell-sap in Plants and their Physical Evironment,’’ Bio. Chem. Jour., vol. rv, 1907.) The relative acidity of the sap of two plants appeared to be of no impor- tance in the determination of their capacity to form a nutritive couple. Such plants as Carnegiea undergo rapid oxidation on injured surfaces and form wound-cork so rapidly as to inhibit parasitism, except by species with extremely high osmotic activity, which suffer depletion of their own water-balance very slowly and which take solutions from an enforced host against great resistance. Agave as a xeno-parasite forms roots so profusely as to destroy the tissues of the host. Experimental arrangements of xeno- parasites were most successful when regenerated cuttings were inserted in the bodies of the host in a resting condition, in the colder season, with the concentration of the sap increasing, but before the osmotic activity had reached its maximum. THE ORIGINATION OF PARASITISM.* The whole tissue of the body of knowledge organized under biological science is interwoven with the conception that organisms in the march of their evolutionary development make purposeful modifications and display direct reactions to external factors, as a result of which they engage more intricately with environmental components and perform the simple and derived functions with greater efficiency. Such teleological ideas lead the van of a host which come forward under the banner of ‘‘adaptation,’’ a term so diversely applied as to have become well-nigh useless in critical discussions. This is apparent at once when it is realized that the result of a gradual development of a character in any direction by reason of the selection of minute fortuitous variations may also be termed ‘‘adaptations,”’ especially by those naturalists who depend upon natural selection to ac- count for all evolutionary progress. The present discussion concerns the physiologic and morphogenic nature of the alterations which organisms undergo in response to environment, and the whole group of questions as to methods of acquisition, progres- sion, or retrogression fortuitously, purposefully, orthogenetically, contin- uously, or discontinuously, may be held in abeyance for the moment while an attempt is made to show not what living things must do, but what they may do under certain specialized conditions. The accommodation of an organism to unusual concentrations of the solutions in the medium or substratum, and the more obvious response to aridity, humidity, and intensities of radiation have been the subject of numerous tests in the laboratory, and carry the general interest that attaches to results which widen the known capacity of organisms with respect to one of the most primitive and perhaps the most essential property of living matter—the power of adjustment to environment. The changes constituting these reactions are as inevitable, though not quite so direct, as the melting of ice under heat; they are incidents in autogenetic history and may be reversible in the individual, as water may be frozen again, unless accompanied by morphological maturations. Even when individually fixed, transmission of effects beyond a few generations has not been demonstrated. With reference to their evolutionary value it is to be said that such accommodations may be inhibitive to important func- tions and deleterious if preserved, while, on the other hand, it is obvious *The principal conclusions embodied in this paper were discussed in part before the Sigma Xi Society of the University of Chicago, December 9, 1909; the Scientific Soci- ety of Johns Hopkins University, December 20, 1909; and the American Naturalists, Boston, December 22, 1909. 50 THE ORIGINATION OF PARASITISM. 51 that many features constructively adaptive may not be referred to the fac- tors with which they interlock as causal agencies. The literature descriptive of structures and functional arrangements which play some special part in the life of the species under the term ‘adaptations’ is voluminous. Purpose, usefulness, and direct adaptive reactions are described in every striking device that comes to the attention of the observer. A writer may recognize the futility and inadequacy of the teleological interpretation of organisms and still use its forms of reasoning in text-books, lectures, and teaching, because it offers a short, easy method of getting in touch with an audience, a practice open to much objection. Absolute candor compels us to admit that between the individually ac- commodative or ontogenetic responses which might be carried over partially to two or three generations and the complex mechanisms characteristic of species which engage our attention as ‘‘adaptations,’’ so called, there is no definite connection, though we are willing to consider and to give the utmost weight to circumstantial evidence bearing on the matter. The single link which connects direct action of external agencies upon the organisms with permanent changes are those in which climatic factors, unusual forms of radiation, and solutions have shown to affect the germ- cells in such a manner as to bring about permanently heritable alterations. Such changes are not adaptive, although some of them result in greater fitness to new environments, as has been proven in comparative tests of parent and derivative in a series of habitats. These results, however, do not justify the conclusion that all fitness has resulted from selection and survival of germ-variations. (See recent views of L. Cuénot on Origin of Species by Mutation, Science, vol. xxx, p. 768, 1909.) That somatic accommodations and adjustments may be transmissible, the theory of inheritance of acquired characters, the solace of the systematist, is a very real possibility to the physiologist, as many facts suggest that the matter may depend upon combination of circumstances not yet uncovered. The earlier results obtained by the establishment of acclimatization cul- tures in connection with the Desert Laboratory justified the belief that the subject was one amenable to direct experimentation, and that it might be possible to obtain facts of importance by two different methods. One would consist in testing the morphogenic and physiologic reactions of plants to the action of climatic factors, especially with regard to their her- itability. The second method of promise, which should be supplementary, would involve the consideration of arrangements ordinarily termed adapta- tions, of which various stages might be found in accessible types or species. The induction of such adjustments in organisms not yet displaying them, or the accentuation of the characters implied in forms in which they were already present, would be taken as evidence of first-class importance. The lack of positive evidence as to the origination and perpetuation of adjustments making for increased fitness is not to be regarded as singular 52 THE ORIGINATION OF PARASITISM. or as indicating absence of the spirit of inquiry. The inheritance of ac- quired characters has been the subject of much observation and some experi- mentation, but in general the problems in this and in the study of adaptive arrangements are involved, and the experimental tests demanded are diffi- cult of organization and necessarily require periods of time for their com- pletion which would include a large proportion of the active period of any individual worker. Furthermore, it is to be seen that experimentation of the kind demanded in such work must be carried on in field laboratories, or in places where the biotic elements may be handled under habitual conditions of environ- ment. The tendency of the naturalist to carry his tools, his laboratories, and his apparatus to the home of the organism he wishes to study is one which is gaining in favor, and rightly so, since it will undoubtedly yield results of importance on many subjects which have eluded us hitherto. The ethnologist expects and gains but little by examining a member of a tribe hundreds of miles from his habitat and his people, but makes his profitable studies of the groups of human animals in all of their undisturbed relationships with their environment. The biologist must follow him in this if he expects to make material progress in interpreting habit and in formu- lating conceptions of adjustment by morphological differentiation, physio- logical specialization, and acquisition of habital peculiarities. The present occasion may be profitably devoted to the consideration of work with specializations displayed by existing types. After some search for opportunities of promise, attention was directed toward a study of dependent nutrition among seed-plants. The few observations which had been made as to incidental or adventitious parasitism, in which the roots of ordinarily autophytic green plants fastened on the underground mem- bers of other plants, the investigations of Cannon, in which the parasitism of Krameria upon various species was discovered, and the experiments of Peirce in artificial parasitism have led to the selection. It is known that organic matter may enter the absorbing organs of nearly all plants and that many of the unions of two species made in grafting operations result in parasitism for the cion, especially when the cion and stock represent widely divergent morphological types. All of these facts show that the tendency to parasitism is very strong among plants, and that a movement towards its accomplishment might be made which would carry a species rapidly through a wide range of adjustment. That this movement has affected a notable share of the higher plants is evidenced by the fact that parasitism is widely prevalent in nine great fami- lies or orders, and the discovery of Cannon adds to this a tenth, the Krame- riaceee. One genus in every 200 includes parasites, and most impressive of allis that about 2,500 species of parasitic seed-plants are known—about 2 per cent of the recognized forms—and a thorough examination would doubtless double this number. A much wider category of plants is THE ORIGINATION OF PARASITISM. 53 included in the mycorrhizal forms, in which symbiotic arrangements between fungi and roots result in forms of nutrition accompanied by alter- ations in the shoot of the higher plant separable in neither degree nor kind from those characteristic of parasitism. Marked developments of mycorrhizal arrangements are known in a number of species comparable to parasites, but the instances in which they are present in some incipient form are so many that it would be justifiable to say that the adaptive move- ment or adjustment by which higher plants use organic material derived from other organisms is one which is manifested by perhaps half their total number, making it evident that the feature under discussion is one of the most important in the evolutionary development of the vegetable kingdom. The work being done upon the investigation of the water-balance of desert plants suggested that these forms, particularly the succulents, might well furnish objects advantageous for experimentation upon the subject. The reservoirs of material in solution held by such plants would be expected to furnish ready places of attack for possible parasites and, on the other hand, various studies, the most recent by Kusano, show that parasites, particularly the Rhinanthacee and Santalacee, carry a large water-balance in specialized tracheids. (Kusano, S., on the Parasitism of Siphonostegia, Bull. Coll. of Agric., Tokyo Imperial University, vol. v1, No. 1, 1908.) A water-balance is, therefore, in some cases at least, an accompaniment anda condition of parasitism, and advantage was taken of the presumption that its presence would be an experimental advantage. In the consideration of conditions which might operate to bring two plants together in a nutritive relation, it was seen first of all that a cer- tain general coincidence of habit was a prerequisite, in illustration of which it may be pointed out that a slow-growing perennial could not flourish or survive if attached to the body of an annual, or that a winter annual would hardly fasten to the quiescent body of a perennial showing activity in growth and food formation only in the summer. Furthermore, the oppor- tunity for the formation of absorptive structures in contact with a possible host would be of importance. Roots submerged in moist soil would naturally afford the best conditions for such action, and the greatest number of cases of parasitism among the higher plants are of the type in which under- ground organs are united. The adhesion of shoots would not occur so profusely, except as consequent upon the mechanical dependency of the climbing or the twining plants, and hence it represents an advanced spe- cialization when compared with the simpler forms of root adhesion. Cus- cuta offers itself as a well-known illustration of this type of developing parasitism. The epidermal structures—coatings and bark—and the capacity of the cortex for the formation of wound-tissues or dense secretions are factors of much moment in nutritive couplings, as has been amply demonstrated by features of my own work which can not be described here. It is evident 54 THE ORIGINATION OF PARASITISM. that a parasite must withdraw material from its host in accordance with the osmotic power of its juices. The relative concentration or freezing-point of the sap of two plants would, therefore, be a factor in the determination of their meeting in a parasitic union. This is undoubtedly an ultimate limiting condition act- ing as a barrier which all but closes the narrow gateway through which two seed-plants must meet if they are to assume the relations of host and dependent. It will now be in order to discuss the results of experimental arrangements of plants with respect to these operative factors and limit- ing conditions. The material employed included Echinocactus (the melon cactus), Car- negiea (the tree cactus or sahuaro), and various species of Opuntia as en- forced hosts, and two Mexican grapes or Cissus, Tradescantia, Agave, and various cylindrical and flat-stemmed opuntias as ‘‘“xeno-parasites,’’ as it is proposed to designate them. Sections of stems a few inches in length were prepared and allowed to regenerate the wounded surfaces, with the formation of preliminary root- extrusions on some cases. Cavities were then made in the bodies of the hosts, and the bases of xeno-parasites inserted. In some cases the turgid tissues rich in mucilage would close around the inserted slip in such man- ner as to seal the preparation effectively, while in other cases gelatine, wax, and plaster were employed for the purpose of holding the would-be parasite in place and preventing too ready access of air. Many hundreds of preparations were made, some of which were so successful that the xeno- parasite has successfully maintained itself for two years in connection with the host, making new internodes-and leaves in a characteristic manner. The length of time over which the work has been carried permits a fair analysis of the adaptive adjustments which have been called into existence experimentally. First, it is to be seen by a casual inspection of the experimental results that the osmotic activity of the sap and its variation throughout the seasons is a matter of the greatest importance in the approach of two forms to the position of host and dependent. Cissus, with an osmotic activity of 11.34 atmospheres (ash in 100 c.c., 1.39) was successfully parasitic on Opuntia blakeana at 8.88 atmospheres (ash, 1.15), but less so on Echinocactus at 5.72 atmospheres (ash, 1.20), and not all on Carnegiea at 6.78 atmospheres (ash, 1.00), the failures in the two last-named being due to other condi- tions than absorbent capacity. Opuntia blakeana at 8.88 atmospheres (ash, 1.15) was successfully parasitic on Echinocactus at 5.72 atmospheres (ash, 1.20). Opuntia versicolor at 11.98 atmospheres (ash 2.10) main- tained itself upon O. d/akeana at 8.88 atmospheres (ash, 1.15), upon Carnegiea at 6.78 atmospheres (ash, 1.00), and Echinocactus at 5.72 atmos- pheres (ash, 1.20). Agave at 10.84 atmospheres (ash, 1.096) was suc- cessful in maintaining existence on Opuntia blakeana at 8.88 atmospheres THE ORIGINATION OF PARASITISM. 55 (ash, 1.15). Zchinocactus at 5.72 atmospheres (ash, 1.20), and Carnegiea at 6.78 atmospheres (ash, 1.00), for two seasons or more. It is obvious that the form which showed the greatest depletion of its water-balance would be least liable to afford support for a parasite, and no species has been seen to maintain itself on a host with superior osmotic power, but in consideration of this matter other conditions are to be taken into account. Among these is the capacity of accommodation displayed by some plants, in which the encounter of the absorbing organs with a superior concentration of solutions in the medium is followed by an auto- matic increase in the osmotic activity of the cell-sap in the absorbent cells, which might or might not be participated in by the whole body. Detection of this fact in algee and fungi, its demonstration in the root-hairs of salt-marsh plants by Hill (Observations on the Osmotic Properties of the Root-hairs of certain Salt-Marsh Plants, The New Phytologist, vol. vir, p. 133, 1908), the discovery by Livingston that desert plants might sustain themselves for a time in soils which showed an absorbent power for water superior to that of the plant, and the results of Peirce, who determined that peas grown on vines made artificially parasitic on Vicia had a higher osmotic activity than that of peas taken from plants normally nourished, constitute evidence of the greatest interest in this connection. Such automatic vari- ations would undoubtedly be an important factor in periods of extreme desiccation of a host. It was thought that acidity of the sap might be a possible factor and that a parasite could not sustain itself on a more highly acid host. The most successful xeno-parasite is Cissus, with an index of 215, which grew on Opuntia (120) and Echinocactus (89) for extended periods, and on Carnegiea (151) with less success. On the other hand, Agave (104) and Opuntia (120) grew on Carnegiea (ranging from 151 to 188). The cylindrical Opuntia (483), much more highly acid than the prickly pears, was suc- cessfully parasitic on Carnegiea (151) and Echinocactus (89). Relative acidity must be reckoned as only a minor factor in view of these facts. The insertion of Cissus which proved parasitic, on Opuntia developed adventitious roots which pierced the tissues of the host. Agave likewise developed adventitious roots, which in some cases were so numerous as to destroy great tracts of the host. Opuntia used as a parasite did not de- velop roots, but remained with living epidermal cells in more or less direct contact with the epidermal or cortical tissues of the host, exhibiting much the same behavior as certain plants in grafts. Here, then, are two forms of parasitism, one in which roots were developed and another in which they were not. That it is not necessary to develop root-organs for absorption is shown very clearly by Cuscuta, where the papillar extrusions of epider- mal tissues were seen by Peirce to carry on absorption before the haustoria took form or showed differentiation of tissues. 56 THE ORIGINATION OF PARASITISM. In the valuation of the arrangements in which some xeno-parasites developed roots and others did not, it is evident that neither would be indicative of greater facility for the assumption of parasitism, since either roots or stems might well give rise to specialized absorptive tissues. The formation of adventitious roots on aerial stems would serve to pierce the tissues of a possible host, but any notable development of these organs would be impossible by reason of the smothering action of the dense tissues, which would yield but little oxygen in comparison with the amount to be obtained in soils or natural waters. Any closely fitting adjustment would necessitate the origination of either new or highly modified structures for absorption from the tissues penetrated. The absorptive structure formed by the single individual of Passifora which has been seen by Peé-Laby to be parasitic on ZLuonymus was formed from the base of a stem, but it is clearly not a root in structure and may be fairly interpreted to be a haus- torium, or new organ, although its discoverer gives it a cauline character. The results of Cannon show that AKvrameria attaches itself to some of its numerous hosts by means of direct prolongations of the root-tips which penetrate the tissues, while the attachment to others is by lateral forma- tions on roots which have the character of new organs. The assumption of a dependent relation by a plant is accompanied by very marked and direct alterations from the normal, which affect every stage of ontogeny of the individual concerned. The first and most appar- ent deviation is one of stature, as individuals parasitically nourished are uniformly smaller than the average independent individuals of the same species. The root-system is less extensive and is sparsely branched; the shoot, likewise, is not so tall nor so densely branched, while a very nota- ble reduction of leaf-surface ensues. Practically all of the organs show measurements less than the normal. The alterations in question fore- shadow, in the first individual subjected to dependent nourishment, the degradation or atrophy which marks the soma of the advanced parasite, and the probability of inheritance of acquired characters is brought before the experimenter with peculiar vividness. No data are at hand to show similar effects of an abundance of organic nutriment in the food-material taken in by ordinary methods of absorption, and it would be unjustifiable to assume such results. The comprehensive evidence bearing upon the influence of cion and stock upon each other obtained by Mitosch, Guignard, Riviére, Daniel, Bailkache, Timpe, Baur, Edler, Winkler, Voechting, and others (see ré- sumé by MacCallum, ‘‘The Reciprocal Influence of Scion and Stock,’’ Plant World, x11, p. 281, 1909), shows conclusively that the alterations in par- asites are not the simple direct result of decreased nutrition alone. The dependent member of a couple may undergo changes in form and struc- ture of its organs, alterations in ash or cellulose content, and show variations in the balance of almost any food-material present, and may receive sub- THE ORIGINATION OF PARASITISM. 57 stance from the host wholly foreign to it. These deviations from the normal are accompanied by functional performaices, in respiration, photo- synthesis, and various types of catalytic action of unusual rates indicative of serious correlational disturbances. The movements displayed by the etiolated and greened slips of Opuntia which were used in the experiments were marked and long continued and are not surpassed in interest by anything revealed by the present investi- gation. These shoots were allowed to continue in an etiolated condition for several months, thus allowing the non-differentiated tracts of tissue to undergo some of the changes toward maturity; and then after these were so well forward as to admit of but little change, were exposed to the light. The thin, flattened stems were flexible and immediately began to give out branches of the usual broadly ovate type of the normal plant. Being now established in parasitic relations with Achinocactus, in which no root- systems were formed, a novel form of nutation was set up, not accompanied by growth, since the shoots displaying it did not increase in length when calibrated to 0.1 mm. Furthermore, the curvatures were as pronounced in the older basal portions of the stems as inthe younger. These curva- tures appeared in one flank of the stem at the tip, and this zone would then slowly slide down to the base, after which it would be transferred to the opposite flank and pass upward to the apex, the total cycle including about three days. At present no such movement has been seen in similar stems rooted in the soil, though few specimens have been examined. If simi- lar movements in any other plants have been found, their record has not been seen by the author. These movements are seen to be produced by the action of the ‘‘cortical’’ parenchyma, which makes up two-thirds of the shortest diameter of the stems. Shortly after the attainment of full size, decortication is begun by shrinkage of this parenchymatous tract, with the result that the walls become various-folded and the epidermis wrinkled. The reduction of the turgidity and the collapse of the cells takes several months, and it is dur- ing this period that the movements are manifested. It can not be deter- mined definitely whether the curvatures are produced by the formation of zones of increased turgidity on the convex side or by lessened pressure on the concave side. The inequality is sufficient, however, to carry the tips of the stems through an arc of 180° with a fairly regular period of about 70 to 72 hours. This movement is illustrative of the reactions which bear no direct or accommodative relation to the factors producing them. In any attempt to summarize the facts bearing upon the entire subject of dependent nutrition as an adjustment or ‘‘adaptation,’’ it is first to be recalled that the couplings of portions of the shoots of species of marked morphological unlikeness in grafting operations often result in adhesions, in which the adjustment of nutrition constituting partial parasitism is initiated in its least pronounced form. 58 THE ORIGINATION OF PARASITISM. Next in importance in the history of investigations upon the matter are the cultures in which seedlings of green plants have been made to send their roots into the bodies of other species as into a substratum. Wheat has thus been nourished on potato tubers for extended periods, and peas have sent their root-systems through medulla of stems of the horse-bean (Vicia) in such manner as to obtain sufficient material to carry out com- plete development. The reductions in both root and shoot members in this case are indicative of a marked parasitism. Finally, there comes up for consideration the experimental arrangements which justify the present paper, in which regenerated shoots of several species were established in nutritive contact with various hosts. The de- velopments of the shootswere of the restricted character shown in parasitism; the roots, when present, were likewise atrophied, and in some instances these organs were entirely lacking, absorption being carried on through epidermal tissues of the bases of stems immersed in the bodies of the hosts. Some of these xeno-parasites lived for extended periods, sustain- ing dependent nutritive relations upon enforced hosts, a few being now at the close of the second year of their existence. In addition to extending the range of experimental research upon the matter, the number of pre- parations employed has permitted some analysis of the conditions which affect parasitism and which must be taken into account in a consideration of its origination and development among the higher plants. The factors of importance are seen to be: coincidence of seasonal cycle, in which extending roots or elongating shoots may encounter active stems and roots of a possible host; supplementary roots and stem habits; succulency or presence of some balance of solutions in the bodies of both possible host and parasite; unequal osmotic activity of the cell-sap of two plants of a possible couple, and equivalent range of variation with regard to this feature, it being obvious that any plant would soon free itself from a de- pendent which did not follow it in its extreme concentration, thus entail- ing, of course, the power of automatic adjustment in the osmotic activity of the dependent, as has been suggested. With regard to anatomical and mechanical features, the ready formation of wound-tissues and copious excretions of mucilage or formation of oxidases would act asa deterrent to a possible parasite, while the penetrative power of submerged or aerial roots and the formation of haustoria or of epidermal absorptive cells would be an opposed factor in the possible parasite. It is thus to be seen that given any two plants, knowledge of their capability for entering into a nutritive couple may be put in the form of an algebraic equation the reduc- tion of which should indicate with some certainty the possibility of their adhesion. The unceasing distributional movements of plants would operate to bring under test conditions a large number of pairs of species, and it seems THE ORIGINATION OF PARASITISM. 59 quite reasonable that new parasitic unions are being constantly formed in almost all kinds of habitats. This fact might escape detection by ordinary methods of observation almost indefinitely. The results of this and previous studies allows us to recognize the limit- ing factors, the operative conditions, and some of the facts as to the physio- logical nature of parasitism. Few specializations present their main features so clearly to the student. When the total body of evidence, how- ever, is evaluated in the light of current theories, it is found that it is no easy matter to decide by what main methods of evolutionary procedure this adjustment or ‘‘adaptation’’ is attained and how it is advanced from stage to stage. Some alterations, such as that of PasstHora on Fuonvmus, are distinctly discontinuous, but in this particular instance it can not be shown that any permanent results would follow the occurrence of the dependent relation. Some features, however, such as the development of haustoria on roots and stems, would be a distinct mutation and is well illustrated by Krameria and supposedly also by Cuscuta. The taking on of absorptive functions by the epidermal cells of xeno-parasites as described in the present paper is to be recognized as a distinct mutation. A direct and immediate atrophy of the various organs of the shoot ensues as a result of the assumption of the parasitic relation, but the ex- treme stages of such reductions appear to have been reached by gradual changes, although it is evident that such a conclusion is almost wholly inferential. Thetransition from autophytism to complete parasitism, with attendant habital characters, appears to have been gradual, since species may be cited to illustrate all degrees of the alteration; but on the other hand it is not impossible that the complete change may have been made at once in some forms. Nothing in the entire matter suggests progression or retro- gression of all of the involved characters by one method alone. Viewed from another angle, it may be seen that some of the alterations described may be taken to be directly interlocking, or reactive, essential, and practically irreversible. Correlated with these, having no direct con- nection with originating external causes or limiting conditions, but inev- itably consequent upon the primary alterations, are a number of secondary changes which may be the most obvious, but in reality of lesser importance. Modifications of the absorbent organs and of the nutritive systems would be included in the first, while incidental atrophies and other characters, such as the striking nutatory movements described, would be examples of the second. The ingenious and intricate and strained interpretations made to include all of the phenomena displayed by organisms which bear special- ized relations to other organisms or to environmental factors are in strik- ing contrast with this view. A good mechanical illustration of the alterations of an organism, in its adaptive adjustment to any factor, is offered by the behavior of a drop of 60 THE ORIGINATION OF PARASITISM. liquid, such as water, when allowed to rest upon a rough surface which it will not wet. Under equable external conditions such a drop would be approximately globular, with all of its sectors practically equivalent (fig. 2,.4). When placed upon a roughened surface the shape of the drop would be altered as shown in fig. 2, B, the sectors in contact with the hard external body would be markedly modified, the position of the centroid would not be the same as that of the drop originally, and the non-engaged sectors would be altered to a degree corresponding to the directness of their connection with those in contact with the disturbing objects. Fiaq. 2.--d, diagram showing general equivalence of sectors of a drop of liquid in a homogeneous medium. 8B, condition of drop when broughtinto contact with a rough surface, showing greatest distortion in sectors in direct contact, and least in others by correlation stresses. With the further application of this illustration the study of the mode of adjustment of an organism to an environmental factor would entail a de- termination of the changes in the engaging sectors or characters directly modified, the measurement of the effects of the external factors, an estima- tion of the limiting conditions, and the detection of connected variations in the other sectors, functions, or qualities of the living drop. Among the many other suggestive parallels that might be drawn is to be mentioned the mechanical fact that under the altered conditions of surface-tension ensuing from contact with one set of hard objects, the drop is much more liable to be changed further or to be broken up by other agencies acting upon the free or unengaged surfaces. PEATE S A. Cissus digitata parasitic on Opuntia blakeana. B. Section showing development of roots of Cissus in Opuntia. C. Base of leaves and roots of Agave developed as a Xeno-parasite on Opuntia. PLATE 6 Cissus digitata parasitic on Opuntia blakeana. Xeno-parasite has formed a vine several centimeters in length. Long aerial roots have been formed. PIRATES A small plant of Opuntia blakeana with several branches developed in constant temperature dark room. Four detached etiolated branches of O. discata ready for exposure to light and for propagation are shown. PLATE 8 “o0Q] Y8no1y) quewesoul pedejdsip apeds ayy jo uo url suo ay] “eueaye(q eqUNdG Jo s}ooys UseJ8 poyejorje Om] pue ‘10]OI1sI9A enundg omy ‘oae8yy ue ssoysered-ousy se BuLIeeq snyjoRooUTyoy uy PLATE 9 ee f summit. tia in cavities o Sahuaro (Carnegiea gigantea) with Opun' PLATE 10 A, Opuntia blakeana growing on trunk of Parkinsonia microphylla, with the root-system shown in section. B. Cissus as a Xeno-parasite on Echinocactus. ae “ le aay a oi i Mectanen Ponty ) ss at NY eaee an Hi i" ny anh an Att RN eTA nh oat TAC TR } a ee iy PAT ie i i i ti iM