ALBERT R. MAJ:^In 
 
 LIBRyvRY 
 
 AT 
 
 CORNELL UNIVERSITY 
 
CORNELL UNIVERSITY LIBRARY 
 
 3 1924 062 873 108 
 
ENVIRONMENTAL INFLUENCES 
 ON NECTAR SECRETION 
 
 BY LESLIE ALVA KENOYER 
 
 AGRICULTURAL EXPERIMENT STATION 
 
 IOWA STATE COLLEGE OF AGRICULTURE 
 
 AND MECHANIC ARTS 
 
 BOTANY AND CHEMISTRY SECTIONS 
 
 RESEARCH BULLETIN NO. 37 
 
 NOVEMBER, 1916 
 
 AMES, IOWA 
 
Cornell University 
 Library 
 
 The original of tiiis book is in 
 tine Cornell University Library. 
 
 There are no known copyright restrictions in 
 the United States on the use of the text. 
 
 http://www.archive.org/details/cu31924062873108 
 
November, 1916 Research Bulletin No. 37 
 
 ENVIRONMENTAL INFLUENCES 
 ON NECTAR SECRETION 
 
 BY LESLIE ALVA KENOY ER 
 
 AGRICULTURAL EXPERIMENT STATION 
 
 IOWA STATE COLLEGE OF AGRICULTURE 
 
 AND MECHANIC ARTS 
 
 Botany and Chemistry Section 
 
 AMES, IOWA 
 
O^TTCERS AND STAFF 
 IOWA AGRICULTURAL EXPERIMENT STATION 
 
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 sistant in Animal Husbandry Husbandry 
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ENVIRONMENTAL INFLUENCES ON 
 NECTAR SECRETION 
 
 By Leslie Alya Kenoyer 
 
 This stiidy was undertaken to summarize and supplement ex- 
 isting knowledge of the factors which stimulate or retard the 
 secretion of nectar. I'he work was carried out under the direc- 
 tion of the botany section of the Iowa Agricultural State 
 Experiment Station in cooperation with the chemistry section, 
 being done mostly at Ames, Iowa, from June, 1914, to June, 
 1916. 
 
 HISTORICAL 
 
 One of the most complete treatises on nectar, with quite an 
 extensive account of some of the environmental factors in its 
 secretion, was given us by Bonnier (1). The subject of secre- 
 tion has been debated from a physical standpoint. Godlewski 
 (9) attributes it to a fluctuation in the concentration of the cell 
 sap due to alternate splitting and recombination of complex 
 molecules. Pfeifer (19) advances three possible causes for se- 
 cretion: First, an unequal permeability of the membrane of 
 the absorbing and excreting portion of the cell ; second, ati un- 
 equal distribution of solutes in the absorbing and excreting por- 
 tions of the cell; third, the transformation into sugar of the 
 outer portion of the cell wall, and the osmotic action of this 
 sugar upon the liquid contents of the cell. Lepesehkin (14) in 
 a study of the coenocytic plant, Pilobolus, finds evidence that 
 the first of Pfeffer's theories is the correct one for the excretion 
 of water drops. Wilson (29) gives evidence in support of 
 Pfeffer's third theory, showing that the thoro washing of a 
 nectary stops the secretion if the nectary is past the stage of 
 metamorphosis of the cell wall, but that secretion is resumed 
 on the addition of sugar to the surface of the nectary. The 
 validity of his results are called in question by Lepesehkin (14) 
 and Btisgen (5). 
 
 Haupt (10) in a study of extrafloral nectaries finds that 
 after washing, some nectaries become inactive, while others-, 
 as those of the leaves of Impatiens parviflora continue excretion 
 of water but not that of sugar, thus becoming equivalent to 
 hydathodes. 
 
 Livingston (16) likens nectar secretion to guttation, account- 
 ing for the latter by decrease in the permeability, of the plasma 
 membrane induced by an increased turgidity, for the former by 
 hypothetical rapid increase in the solute content and thereby of 
 
220 
 
 the osmotic pressure in the cell,— a change which induces a 
 like decrease in the permeability of the membrane. 
 
 Comparatively little work has been done on the chemistry of 
 nectar. Wilson (28), Von Planta (27) and Bonnier (1) have 
 analyzed a few sorts of nectar finding that in some cases it con- 
 tains no sucrose while in others it is almost wholly this kind of ' 
 sugar. In some cases fructose and in others glucose is the dom- 
 inating reducing sugar. The sucrose of nectar is almost wholly 
 digested in honey, Browne (4) finding as the average composi- 
 tion of 138 honey samples from widely separated localities 
 38.65% fructose, 34.48% glucose and 1.76% sucrose. 
 
 EXPERIMENTAL 
 
 I. Methods 
 
 Nectar, when secreted in sufficient quantities, was measured 
 by means of a graduated capillary pipette, or weighed after 
 absorption on strips of filter paper which had been previously 
 weighed in small vials. Many of the most important honey 
 plants secrete such small amounts of nectar to the individual 
 flower that neither of these methods is practicable. In these 
 the amount of sugar external to the nectaries was approxi- 
 mately determined" by adding to a counted or weighed quantity 
 of the flowers a definite volume of water, shaking frequently 
 for a half hour, then decanting. A similar method was em- 
 ployed by Von Planta (27) and Bonnier (2). In some of the 
 flowers investigated, this treatment extracts some sugar from 
 the floral tissues, as shown by the appearance in the solution of 
 colors from the floral envelopes. Hence it is of value mainly 
 for the comparison of flowers of the same species. Buckwheat, 
 because of its rapid m^aturing and its' value as a honey produc- 
 ing crop as well as its comparative freedom the above noted 
 source of error, was employed in many of the experiments. 
 
 Sugar determinations were made by reduction of Fehling's 
 solution. The method found most practicable, and employed 
 for the greater part of the work, was based on that described 
 by Schoorl (24). A carefully measured amount (1 cc. and for 
 minute quantities of sugar, 10 ec.) of the material to be an- 
 alyzed was placed in a 150 cc. Erlenmeyer flask, heated on an 
 asbestos gauze over a flam« so adjusted that the liquid began 
 to boil in just two minutes, then boiled two minutes longer. 
 To the contents of the flask, after cooling to 60° C, were added 
 sulphuric acid and potassium iodide. The liberated iodine, 
 which corresponds to the unused copper sulphate, was titrated 
 against sodium thiosulphate. Sugar values were obtained by 
 the careful analysis of known quantities of sugar. This method 
 has the advantage of being both rapid and delicate enough' to 
 
221 
 
 determine minute quantities of sugar with a probable error of 
 not over .04 mg. 
 
 Floral tissues, when not too bulky, can be analyzed by the 
 same method, the reagents being added directly to the tissues 
 after covering them with water. When tissues were more bulky 
 or when greater accuracy was required, extractions were made 
 with alcohol or water, and were purified by treatment with 
 neutral lead acetate. 
 
 II. Humidity 
 
 It is well known that any watery exudation from plants ac- 
 cumulates when atmospheric humidity is high and evaporation 
 thereby is retarded. This can easily be demonstrated in connec- 
 tion with bleeding from several tissues, or with guttation thru 
 water stomata. Bonnier (1) states that nectar secretion cor- 
 responds to guttation and that it varies inversely with the 
 transpiration. So far as the volume of nectar is concerned this 
 was found to be true in all the plants experimented upon with 
 this end in view. But, as shown by Pfeffer (20), two factors 
 are involved in nectar secretion, the exudation of water and 
 that of sugar. Haupt (10) has found that extrafloral nectaries 
 begin secreting only when humidity is relatively high, an ob- 
 servation which confirms the theory that secretion is due to a 
 decreased permeability caused by increased turgor, but that 
 after secretion begins, increased air moisture increases water 
 secretion, the secretion of sugar remaining constant. It is prob- 
 able that this applies to nectaries in general. Nectar is more 
 dilute when humidity is high, and honey that is stored at such 
 times is likely to be high in water content. 
 
 At Ames the seasons of 1914 and 1915 represented extremes 
 of humidity, the summer months of the former year being 
 excessively dry and warm, while those of the latter were ex- 
 cessively wet and cool. Hence the comparisons in table I of 
 nectar washed from flowers are of interest. 
 
 TABLE I— COMPARISONS OP NECTAR WASHED PROM PLOWERS 
 IN SEASONS OP DIFFERENT HUMIDITY. 
 
 
 1914] (Dry and warm) 
 
 1915 (Wet and cool) 
 
 
 No. of 
 samples 
 analyzed 
 
 Ave. me. 
 
 sugar 
 
 per gm. 
 
 No. of 
 samples 
 analyzed 
 
 Ave. mg. 
 
 sugar 
 
 per gm. 
 
 
 6 
 4 
 4 
 
 2.13 
 1.15 
 3.64 
 
 3 
 
 3 
 
 13 
 
 .65 
 
 Medicago sativa, flowers 
 
 Trifolimn pratense, corollas 
 
 .80 
 3.90 
 
 It is seen that the wet season yielded scarcely as much sugar 
 as the dry. It may be stated further that bee visitors to 
 Melilotus were several times as abundant in 1914 as in 1915. 
 The author found by experiment that flowers of alfalfa grown 
 in dry soil contain about 60% more sugar than those grown in 
 wet soil. 
 
222 
 
 Buckwheat flowers kept humid under a bell jar secreted 
 much more liquid than flowers exposed to the rather dry green- 
 house air. However, 12 comparative analyses of the nectar of 
 each showed in the humid, 1.04 rag. sugar per 10 blossoms; in 
 the dry, .98 mg. sugar per 10 blossoms. Analysis of the flower 
 after removal of the nectar, in 6 of the above pairs of cases 
 showed .74 mg. per 10 blossoms in the humid, and .98 mg. per 
 10 blossoms in the dry. J\Iore sugar accumulated in a dry atmo- 
 sphere and practically the same amount is excreted. 
 
 The accumulation of sugar under low moisture conditions is 
 in line with the discovery by Lundegardh (17) that increase 
 of moisture favors the accumulation of starch, decrease of mois- 
 ture its digestion. 
 
 Six plants of Impatiens SitMani in saturated air accumulated 
 in a day 3.26 mg. sugar each from the extrafloral nectaries, the 
 basal teeth of the leaves, while 6 plants in greenhouse air ac- 
 cumulated 5.42 mg. sugar each. The excess of the latter is 
 very likely due in part to the running away of drops under 
 the humid conditions. The nectar averaged 23.4% sugar in 
 the former and 45.3% of the latter. 
 
 ///. Rainfall 
 
 The author has shown in a statistical study (12) that heavy 
 rainfall just prior to the secreting season is advantageous as 
 it gives the plants greater vigor. But during the season of 
 greatest secretion, good years are somewhat drier than poor. 
 Also, a rainy day shows a lighter honey yield than a day before 
 or after the rain. 
 
 The deterrent effect of the rain on the honey flow is twofold : 
 It hinders the activities of bees and it washes away the nectar. 
 To illustrate the latter point, in 1915 on the morning following 
 a day of continual rainfall, red clover corollas were found to 
 contain .02 mg. sugar per gm., whereas a day earlier they con- 
 tained 3.8 mg. ; a day later, .6 mg., and two days later 4*4 mg. 
 Buckwheat blossoms were subjected to an experiment to deter- 
 mine the extent to which rains wash away the nectar. Flow- 
 ers subjected before gathering to a spray for 20 minutes, 15 
 mm. of water falling were found to contain .12 mg. per 10 as 
 against 1.28 mg. per 10' untreated flowers. A 30 minute rain 
 of 35 mm. reduced the nectar of red clover blossoms from .48 
 to .19 mg. per 10 and that of white clover blossoms from 27 
 to .07 per 10. 
 
 IV. Temperature 
 
 Wilson (29) states that temperature has not a marked effect 
 upon the rate of secretion of nectaries that have commenced 
 secreting. He finds, however, that Prunus Laurocerasus will not 
 
223 
 
 begin secretion unless the temperature is 12° C. or over. Haupt 
 (10) als0 finds that a minimum temperature is necessary to 
 induce secretion. Lepeschkin (14) finds in the hyphae of 
 Pilobolus a secretion steadily increasing with, and much more 
 rapidly than, the absolute temperature. In other cases he 
 finds an optimum above which secretion diminishes. In the 
 case of secreting hairs of the bean leaf this optimum is 20° C, 
 in the Abutilon nectary it is 26° 0. 
 
 Experiments were carried out in uniform temperature incuba- 
 tors. For much of the work, to avoid light exclusion which is 
 detrimental to secretion, incubators were employed which were 
 especially constructed for the purpose, being covered with two 
 glass plates separated by an air space. 
 
 The optimum temperature for amount of secretion lies between 
 20° C. and 25° C. for Cucvrrbita pepo, LiUum speciosnm, Gmma 
 indica, Euphorbia pulcherrima, and extra floral nectaries of 
 Impatiens Sultani. For Salvia splendens and most of the Legum- 
 inosae tested it is about 15° C. As a rule the sugar concentration 
 of the nectar does not differ materially for the different temper- 
 atures. Tables II gives typical sugar determinations obtaftied 
 from the flower of Ahidilon striatum, the blossoms being quar- 
 tered and one piece of each placed in each incubator, thereby 
 eliminating any error due to in-dividual variations. 
 
 TABLE 11— TYPICAL SUGAR DETERMINATIONS FOR 
 ABUTILON STRIATUM. 
 
 Degrees temperature 
 
 Mg. invert sugar per flower. 
 
 10".^ 
 
 16-c 1 23°<i 1 
 
 30°« 
 
 
 10:20 
 3.07 
 
 12.00 16.00 
 6.57 12.97 
 
 10 32 
 
 After 16 hours (another set)'. 
 
 10.87 
 
 Here the optimum is clearly not far from 23° C. 
 
 Bonnier and Flahault 91.80' call attention to the fact that 
 nectar secretion is greater in the same species a^t high latitudes 
 and altitudes than at low when the species grows normally in 
 both latitudes of altitudes compared, and furthermore that 
 species which do not secrete in France are nectariferous in Nor- 
 way and in the Alps. He suggests that this fact may be due to 
 the greater range between maximum and minimum daily tem- 
 peratures whicih prevaflla at high altitudes and latitudes, or to the 
 greater range in the humidity of the air. 
 
 Philips (21) observes- that alfalfa is, as a rufe, valuable as a 
 honey plaat in; the great prairie region of the west and not in 
 the eastern states, that buckwheat is of more value in New York, 
 Pennsylvania and Michigan than in Indiana and Illinois, and 
 that white dover is of greater importance in the north than in 
 the south. Basswood is said to secrete better in the more north- 
 erly portions of its range. It therefore seems desirable to inves- 
 tigate the hypothesis of Bonnier. 
 
224 
 
 As already shown by the author, (12), the study of a 30-year 
 weight record of a hive at Clarinda, Iowa, lends strong support 
 to this assumption. Thirty-eight periods of continual and fairly 
 rapid gain in weight were selected, and the days of each divided 
 about equally between days of high gain and days of low gain. 
 In 32 cases the average diurnal temperature range for the days 
 of high gain is greater than that for the days of low gain. In 
 all of the six exceptional cases the difference between the average 
 is small. 
 
 Sladen (14) states that the heaviest single day's increase in 
 hive weight noted for two seasons in England in a record kept 
 by Dr. Moore Bde was on a day that began with a heavy early 
 morning frost, the honey coming from the heather, Calluna 
 vulgaris. 
 
 Table III represents the amount of reducing sugar in mgs. 
 found after keeping the plants or flowering branches for a time 
 in the incubators. 
 
 TABLE III— AMOUNT OF REDUCING SUGAR IN VARIOUS 
 PLANTS AFTER INCUBATION. 
 
 Trifolium incarnatum, 10 
 
 .56 
 
 .63 
 
 1.12 
 .07 
 
 .63 
 
 
 2.08 
 
 22.22 
 4.80 
 
 .73 
 .80 
 
 26.7 
 
 .86 
 
 45.0 
 17.1 
 
 .43 
 
 .94 
 .02 
 
 .47 
 
 
 .52 
 
 11.95 
 2.00 
 
 .58 
 .0 
 
 13.8 
 
 .78 
 
 19.3 
 11.9 
 
 .38 
 
 .70 
 .01 
 
 
 tig 
 
 
 Trifolium repens, 10 
 
 fls 2 da 
 
 
 
 
 
 
 
 Trifolium repens. 10 
 fls 2 da 
 
 .93 
 
 
 
 3.44 
 
 37.55 
 .64 
 
 .69 
 1.36 
 
 .56 
 .08 
 
 
 
 
 
 Medicago sativa 10 fls. 
 4 da 
 
 
 
 Caragana frutescens 
 
 10 fls 
 
 
 24.33 
 2.88 
 
 
 
 
 
 Fagopyrum esculentum 
 10 fls. 4 da 
 
 
 
 
 
 
 
 Salvia splendens 10 
 
 12.9 
 
 .62 
 
 13.9 
 
 no 
 
 
 
 Coleus blumei 10 
 
 
 
 
 Taraxacum officinale per 
 
 38.7 
 15.6 
 
 
 
 Taraxacum officinale per 
 ■ -gm. fls. 1 day 
 
 t develop! 
 
 
 In field conditions it can readily be shown that lower tempera- 
 tures increase the sugar content in dandelion and the clovers. 
 
 VanKysselberghe (26) determined that with increase in tem- 
 perature the permeability of the protoplast to water and solutes 
 rapidly increases, — that of Tradescantia epidermal cells for 
 
225 
 
 water being eight times as great at 30° as at 0° C, and that for 
 solutes seeming to follow the same proportional rule. To dem- 
 onstrate whether this holds for nectary cells, the lowest sucrose 
 concentration necessary to plasmolyze the multicellular secret- 
 ing hairs which cover the nectary of Abutilon was determined. 
 After four days at 10° C. a .6 molecular solution was found suffic- 
 ient, while for another portion of the same flower after four days 
 at 25° C. a 1.1 molecular solution was necessary. 
 
 In a study of the influence of low temperature Miiller-Thurgau 
 (18) found that sugar accumulates in potatoes when surround- 
 ing temperature is below 10° C, and the same thing is true of 
 hemp seedlings and various other plant organs. He advanced 
 the theory that the accumulation comes from the digestion of 
 starch or oil more rapidly than the resulting sugar can be 
 utilized, — its respiratory destruction being retarded by the low 
 temperature. 
 
 The accumulation of stored sugar from starch in low tempera- 
 ture is a well-known phenomenon in the twigs of woody plants 
 which is amply discussed by Fisher (8). Indeed, this accumula- 
 tion seems to be rather in its occurrence among plant tissues. 
 Besides being applicable to floral tissues, as table III so clearly 
 shows, it affects the leaves and the peduncles of white clover, the 
 former after two days treatment having 30% more sugar, and the 
 latter 58% more, at 10° C. than at 25° C. 
 
 The evidence points to the conclusion that at a uniform temper- 
 ature the secretion of nectar is a balance between two factors, — 
 the accumulation of sugar in and near the flower under the in- 
 fluence of low temperatures and increasing permeability of the 
 plasma membrane under the influence of high temperature. The 
 position of the optimum, then, might be represented somewhat 
 as in the figure below. 
 
 / ^ \ ofhrotoh/a'jrJdJi/aar^ 
 
 Vot/'mc/m secrefJor? femberaA^r/' 
 
 71?mtipratt/rf 
 
 /0° £0' ?0° 
 
226 
 
 The two graphs are limiting factors to nectar secretion and 
 the intersection, i. e., the point where the effetive limit stands 
 highest, is the optimum secretion temperature. If the fact 
 discovered by Bckerson (7) for root cells, that above a certain 
 point (25°-35°) the permeability again decreases, applies also 
 to nectary cells, the situation may be somewhat complicated 
 thereby. 
 
 Better than any uniform temperature for secretion is a change 
 from a lower to a higher temperature, — as the table indicates. 
 The influence of such a change might be graphically indicated 
 by folding the above diagram so that two temperatures, — say 
 10° C. and 30° C.,— are brought together. Both limiting factors 
 are raised; the sugar which has accumulated at the lower tem- 
 perature is secreted at the higher. 
 
 V. Atmospheric Pressure 
 
 In the author's study previously cited (6) it was shown that of 
 18 periods of continual honey production, 16 have a lower bar- 
 ometric pressure on the days of heavier yield than on the days 
 of lighter yield, the two exceptional cases having very slight dif- 
 ferences. The increased secretion already credited to high alti- 
 tudes might b« attributed to the diminished pressure, but this 
 explanation would of course not account for the similar increase 
 at high latitudes. 
 
 In investigating experimentally the influence of pressure, the 
 plant under experiment was covered with a tabulated bell jar 
 waxed to fit tightly to a ground glass plate and connected by 
 means, of a s-top cock with the water aspirator. A similar plant 
 was placed under a control bell jar. In some of the experiments 
 air was daily renewed in both low pressure and control jar ; in 
 others its continual renewal was provided for by admitting a 
 current of air which bubbled thru water and in the case of the 
 low pressure jar, entered by means of a capillary tube with a 
 very small aperture. This latter method is similar to one em- 
 ployed by Schaible (22). By the use of an aneroid barometer 
 the pressure was maintained at about 50 cm. or two-thirds atmo- 
 spheric pressure, the prevailing condition at altitudes of about 
 ten thousand feet. Repeated investigations were made with the 
 following plants : 
 
 For guttation — Tropaeolum ma jus and Avena sativa. 
 
 For nectar secretion — Tropaeolum majus, Impatiens Sultani, 
 Ahutilon striatum, Euphorbia pulcherrima, Canna indica, Fago- 
 pyrum esculentum, Salvia splend.ens, Coleus Blumei, Antirrhi- 
 num majus and Prunus americana. 
 
 There were no constant differences in secretion which could be 
 detected by either physical or chemical means. It is very doubt- 
 ful therefore whether the much smaller variations in pressure 
 
227 
 
 which occur in nature could measurably affect nectar secretion. 
 
 It is a matter of common knowledge among beekeepers that 
 bees are more active when the barometer is low, the warmth and 
 stillness of such periods favoring activity. Hence it seems very 
 probable that any relation between atmospheric pressure and 
 honey flow is to be attributed to the bees and not to the plants. 
 
 VI. Light 
 
 Darwin (6), Wilson (2)9 and Haupt (10) note the fact that 
 the extrafloral nectaries of several species of Vicia are stimulated 
 to aetivtiy by light. The first author adds Lobelia erinus and 
 the last, the Euphorbiaceae, as plants that require the light stim- 
 ulus for secretion. The two latter authors, however, state that 
 in the greater majority of eases, secretion is only indirectly re- 
 lated to light. Haupt found that in most extrafloral nectaries 
 even disturbances in photosynthesis by darkness show their in- 
 fluence on secretion only very slowly. Light in Vicia doubtless 
 increases the permeability of the protoplast, as Lepeschkin (15) 
 has found that it does in the pulvini of Leguminosae in general. 
 
 Schimper (23) found that extrafloral nectaries on the leaves 
 of Cassia neglecta cease their activity in a few days when the 
 plant is kept in darkness or in an atmosphere deprived of carbon 
 dioxide, but that secretion continues when the leaf is in the light 
 and only the nectaries are darkened. 
 
 The author experimented upon both the floral and the extra- 
 floral nectaries of Impatiens Sultani, and it seems clear that the 
 withdrawal of light makes its influence fairly rapidly and very 
 decidedly felt. Table IV gives a typical study of floral nectaries, 
 the measurements being millimeters in length of the part of the 
 spur which contains nectar. The table includes average in- 
 creases over last measurement of the spur in those flowers which 
 were open when the last record was taken, and the average 
 measurement for those flowers which have opened since the last 
 record. One plant was covered with a bell jar, the other was an 
 opaque jar of about the same size. 
 
 TABLE IV— NECTAR SECRETION IN DARK AND LIGHT. 
 
 Days 
 
 Light 
 
 Dark 
 
 
 Gains 
 
 New flowers 
 
 Gains 
 
 New iiowers 
 
 2 
 
 4.4 
 
 21.6 
 
 1.2 
 
 16.1 
 
 3 
 
 4.8 
 
 22 
 
 1.1 
 
 13.6 
 
 4 
 
 2. 
 
 23 
 
 1.1 
 
 15.7 
 
 6 
 
 3.5 
 
 29 
 
 -1.2 
 
 17.3 
 
 7 
 
 5 
 
 20 
 
 .1 
 
 16 
 
 8 
 
 4.1 
 
 21.7 
 
 .5 
 
 14 
 
 9 
 
 3.3 
 
 19 
 
 -.4 
 
 13 
 
 10 
 
 3.2 
 
 22 
 
 -.5 
 
 
 At this time the dark plant had taken on an etiolated appear- 
 ance and new flowers were scarcely developing. Secretion from 
 extrafloral nectaries had practically stopped after 3 days in the 
 dark. 
 
228 
 
 Half the leaves of a plant were covered with black tissue paper 
 which was fastened by means of small brass paper clips, the 
 basal or nectar-secreting teeth only being left uncovered. These 
 leaves secreted very little after the third day, whereas the un- 
 covered leaves of the same plant were uninterrupted in their 
 secretion. 
 
 Buckwheat flowers were gathered at the same time from under 
 light and daik jars and it was found that after two days in the 
 dark, altho the total amount of liquid secretion was not in the 
 least diminished, the proportion of sugar began to decrease, the 
 secretion not tasting sweet nor giving a very positive sugar test. 
 An average of eleven such analyses of the nectar of flowers that 
 had been coverd for from two to eight days gives per 10 blos- 
 soms 1.20 mg. invert sugar in the light and .41 mg. in the dark. 
 Sugar contained in the flowers does not differ greatly, however, 
 in the two cases, there being .79 mg. to 10 flowers from the light 
 and .73 mg. to 10 flowers from the dark. Plants which had been 
 left in the dark for some time continued to secrete less than 
 normal quantities of nectar for a week or more after the removal 
 of the cover. 
 
 That the diminution of sugar is due to the interference with 
 photosynthesis may be shown by removing all the leaf blades 
 from a number of plants. Eleven analyses averaged in mg. in- 
 vert sugar in the nectar of ten flowers, .36 from plants with 
 leaves removed from 3 to 10 days, .85 from the normal plants 
 serving as checks. Seven of the above pairs of eases give for the 
 entire invert sugar content of the flowers, .59 mg. from the 
 mutilated and .92 mg. from the check. 
 
 The same result may be gained by covering all the leaves of 
 the plant with black tissue paper, and comparing nectar with 
 that of normal plants. Here we find as an average of six an- 
 alyses of 10 flowers .23 and .69 mg. invert sugar, respectively, in 
 the nectar, and .83 and 1.40 in the whole flowers. After four 
 days in the dark there is approximately one-fourth as much sugar 
 secreted per flower. 
 
 When the flowers only are covered from the light, they secrete 
 fully as much sugar as those not covered. So the extrusion of 
 sugar IS clearly dependent upon the food reserves of the plant. 
 
 The same relation was found to exist in the Canna blossom 
 Darkening of the entire plant materially diminished the nectar 
 secretion, while darkening the flower cluster alone had no in- 
 fluence upon it. 
 
 Other flowers analyzed, among them being Antirrhimtm majus, 
 Ciocumis sahvus, Salvia splendens and Coleus Blumei, both con- 
 tamed less sugar and also secreted less sugar when kept in the 
 dark Furthermore, the nectar was usually less in volume. 
 Euphorbia pulchernma did not begin secretion in a dark cham- 
 ber, and Abuhlon striatum secreted only very slowly and very 
 
229 
 
 scantily. Even plum branches, which contain supplies of stored 
 food, when developed in the dark have scarcely over half as much 
 sugar in the tissues and atout a fourth as much nectar as when 
 developed in the light. 
 
 VII. Fertility of Soil and Vigor of Plant 
 
 Hunter (11) states that alfalfa yields the greatest amount of 
 nectar under conditions that tend to give it the most vigorous 
 growth, proper heat and moisture upon suitable soil. All of the 
 observations and experiments in this work tend to confirm this 
 for the various plants investigated. Red clovers grown on fer- 
 tilized plots were foiind to contain slightly more sugar and to 
 secrete slightly more as nectar than those on the unfertilized 
 control plots adjoining. 
 
 Bonnier (2) experimented on the secretion of several plants as 
 influenced by different soils. He found that Sinapis alba, Isatis 
 tinctoria and Medicago sativa yield most nectar on limy soil, 
 while Phacelia tanacetifolia does better on clay, and Fagopyrum 
 esculentiim on sand. It is probable that soils which are conduc- 
 tive to greater vigor and more surplus food in the plant are on 
 the whole more favorable for nectar yield. 
 
 "White clover, the leading honey plant in Iowa, collected the 
 same day in the same part of the city gives the following tests -. 
 
 Sugar per gm. flowers in 
 Nectar Flowem 
 
 Stunted by rank growth of weeds 2 15.9 
 
 Stunted by close mowing 6 20.8 
 
 Vigorous 3.7 21.6 
 
 Vigorous plants of buckwheat yield about twice as much nectar 
 as did weak ones in the same bed. The flowers were found on 
 analysis to contain from 20% to 50% more sugar. Plants which 
 had been allowed to dry to the wilting point several times in the 
 course of their growth, and were consequently stunted to about 
 one-half normal height, yielded less nectar, the average of six 
 comparisons being .44 mg. per 10 flowers of the stunted and 1.39 
 mg. per 10 of the normal. Plants grown in a greenhouse in 
 which the temperature was low and which consequently were 
 stunted to about one-third normal height, blooming not until 
 twice the age of normal blooming plants, secreted- practically no 
 nectar. 
 
 It was further noticed that a salvia deeply rooted in the soil 
 secreted better than one that was hampered by a small pot, and 
 that of the former plant young vigorous branches yielded more 
 nectar than old stunted ones. 
 
230 
 
 VIII. Portion of Flowering Period and Age of Flower 
 
 Table V illustrates the relation of nectar secretion to the part 
 of the flowering season in which the flower in question appears. 
 In all eases the compared flowers were collected on the same day, 
 but from patches varying in stage development. 
 
 TABLE V- 
 
 -RELATION OP NECTAR SECRETION TO FLOWERING 
 SEASON. 
 
 
 Early in Bloomine 
 Season 
 
 Late in Blooming 
 Season 
 
 Red Clover 
 
 Sugar in 
 Nectar 
 
 Sugar in 
 Flowers 
 
 Sucrar in 
 Nectar 
 
 Sugar in 
 Flowers 
 
 (Trifolium pratense) . . . 
 Alfalfa (Medicago sativa) 
 
 per gm. flowers 
 
 Buckwheat 10 fls. 
 
 (ave. 4 tests) 
 
 9.3 
 1.5 
 1.62 
 
 20.7 
 38.9 
 .72 
 
 4.6 
 
 .3 
 
 1.09 
 
 18.6 
 26.8 
 .60 
 
 As a rule the younger plant is undergoing more active photo- 
 synthesis and has greater reserves of food to be secreted by the 
 nectaries. 
 
 Kurr (13) makes the statement that secretion of nectar com- 
 mences very rarely before the dehiscence of the anthers ; it is 
 generally most rapid during the pollination period; and it 
 ceases as soon as the fruit begins to develop. Bonnier (1) agrees 
 to this proposition and insists that nectar is simply a manifesta- 
 tion of the surplus food stored in the nectariferous part corres- 
 ponding to an arrest in the development of the organ. In the 
 case of floral nectaries, then, it is most pronounced after the 
 ovary has attained maximum development and before the fruit 
 
 TABLE VI— SECRETION OF NECTAR AND MATURING OP 
 FLOWERS. 
 
 
 Buds 
 
 Mature flowers 
 
 Declining flowers 
 
 
 Invert t Sucrose 
 Sugar 1 
 
 Invert 
 Sugar 
 
 Sucrose 
 
 Invert 
 Sugar 
 
 Sucrose 
 
 Medicago sativa 
 (Alfalfa) per gm. . . 
 
 Melilotus alba 
 (White sweet clo- 
 ver) per gm 
 
 32. B 
 
 22.2 
 
 16.0 
 
 8.1 
 
 *13.8 
 
 17.8 
 
 47.5 
 
 119.5 
 
 51.4 
 95.9 
 
 2.8 
 2.6 
 
 .1 
 
 * 
 
 56.4 
 
 31.1 
 
 28.8 
 
 8.3 
 
 •8.6 
 
 27.2 
 248.4 
 179.3 
 
 952" " ' 
 
 
 trace 
 
 1.8 
 
 + 
 
 26.1 
 
 
 
 Melilotus officinalis 
 (Yellow sweet clo- 
 ver) per gm 
 
 
 
 Trifolium repens 
 (White clover) 
 per gm 
 
 7.5 
 
 * 
 
 
 Trifolium hybridum 
 (Alsike clover) 
 
 
 Taraxacum offici- 
 nale (Dandelion) 
 per gm 
 
 
 Impatiens Sultani 
 
 
 26.7 
 
 • 14.9 
 21.3 
 
 
 10.9 
 
 11 3' " " 
 
 190.0 
 76.0 
 
 
 Lilium speciosum 
 
 rubrum per flower . . 
 Lilium longiflorum 
 
 per flower 
 
 Younger 
 
 Older 
 
 48.9 
 1.0 
 
 ♦Including sucrose. 
 
 
 
 
 
 
 
231 
 
 has commenced to develop. He finds a maximum proportion of 
 sucrose in the floral organs corresponding to this time of greatest 
 secretion. Chemical analyses of the floral tissues show that the 
 climax of sugar accumulation is about the time of the dehiscence 
 of the stamens, and that as the flower withers there is a very 
 rapid decrease in the amount of sugar. Table VI gives some ex- 
 amples from my work, the tissues having been extracted and 
 purified. 
 
 It seems that in most cases the excess of sucrose, the storage 
 form of sugar is rather before the flower opens and nectar secre- 
 tion begins. 
 
 Recognition is due Drs. Pammel and Dox and Prof. Coover 
 of Iowa State College, Drs. Cowles and Crocker of the Univer- 
 sity of Chicago, Dr. Phillips of the Bureau of Entomology, and 
 Mr. Pellett, bee inspector for Iowa, for encouragement and as- 
 sistance in this work. 
 
 Summary 
 
 1. By increasing humidity, the secretion of water, but not 
 that of sugar, from nectaries is increased. 
 
 2. Excessive water supply lessens the sugar surplus in the 
 parts of the flower. 
 
 3. Dilution and washing by rain causes much of the sugar of 
 nectar to be lost. 
 
 4. Rate of secretion for both sugar and water increases with 
 temperatiire up to a certain optimum. 
 
 5. Accumulation of sugar in the flower and its vicinity varies 
 inversely as the temperature. 
 
 6. The optimum condition for sugar secretion is an alterna- 
 tion of low and high temperatures. 
 
 7. Variation of atmospheric pressure has no marked influ- 
 ence on secretion. 
 
 8. Sugar excretion is markedly diminished in darkness on ac- 
 count of limitation of the food reserves of the plant. 
 Water excretion may or may not continue, depending on the 
 species. Removal of the leaves has the same deterrent effect. 
 
 9. The more favorable all conditions for growth and the more 
 vigorous the plant, the greater is the amount of sugar 
 secreted. 
 
 10. Nectar is most abundant early in the blooming season, other 
 things being equal. 
 
 11. Accumulation and secretion of sugar is most pronounced 
 near the time of the opening of the flower. 
 
232 
 LITERATURE CITED 
 
 1. Bonnier, G., Les Nectaries. Ann. des Sci. Nat. Bot. 1 : 1-212. 1879. 
 
 2. Bonnier, G., Influence du terrain sm la production du nectar des 
 plantes. Assoc. Fr. pour 1' Advancement de Science. P. II: 
 567, 569. 1893. 
 
 3. Bonnier G. & Flaliault, Ch., Observations sur les modifications des 
 vegetaux suivant les conditions physiques du milieu. Ann. des 
 Sci. Nat. Bot. 7: 108-113. 1878. 
 
 4. Browne, C. A., Chemical analysis and composition of American 
 honeys. Bull. U. S. Bur. of Chem. 110. 
 
 5. Biisgen, M., Der Honigthau. Jenaische Zs. Natw. 25: 339-428. 1891. 
 
 6. Darwin, C, Cross and self fertilization in the vegetablef iMngdtfifit 
 Chap. X. 1877. 
 
 7. Eckerson, S., Thermotropism of roots. Bot. Gaz. 58: 254-263. 1914. 
 
 8. Fischer, A., Beitrage zur Physiologie der Holzgewachse. Pring- 
 sheim's Jahrb. fiir Wiss. Bot. 22: 73-160. 
 
 9. Godlewski, E., Zur Theorie der Wasserbewegung in den Pflanzen. 
 Jahrb. f. Wiss. Bot. 15: 602. 1884. 
 
 '10. Haupt, H., Zur Secretionsmechanik der extrafloralen Nektarien. 
 Flora 90: 1. 1902. 
 
 11. Hunter, S. J.. Alfalfa, grasshoppers, bees. Bull. Dept. Bnt. Univ. 
 of Kans. 1899. 
 
 12. Kenoyer, L. A, The weather and honey production Bull Iowa 
 Agrl. Exp. Sta. 169. 1917. 
 
 13. Kurr, 0. G., Bedeutung der Nektarien in den Blumen. Stuttgart 
 1883. 
 
 14. Lepeschkin, W. W., Zur Kenntniss des Mechanismus der aktiven 
 Wasserausscheidung der Planzen. Beiheftez. Bot. Cent. 19: 1: 
 409. 1906. 
 
 15. Lepeschkin, W. W., Kenntniss des Mechanismus der Variationsbe- 
 wegungen unter der Einwirkung des Beleuchtungswechsels aut 
 die Plasmamembrane. Beih. z. Bot. Cent. 24: 38-356. 
 
 16. Livingston, B. E., The role of diffusion and osmotic pressure in 
 plants. 1903. 
 
 17. Lundegardh, H. Einige Bedingungen der Bildung und Auflosung 
 der Starke. Jahrb. Wiss. Bot. 53: 421. 
 
 18. Miiller-Thurgau, H., Ueber Zuckeranhaufung in Pflanzentheilen in 
 Folge niederer Temperatur. Landw. Jahrb. 11: 751. 1882. 
 
 19. Pfeffer, W., Osmotische tJntersuchungen. 1887. 
 
 20. Pfeffer, W., Studien zur Energetik der Pflanze. 267 1892 
 
 21. Phillips, 15. F., Beekeeping. 207, 362. 1915. 
 
 22. Schaible, F., Physiologische Experimente ueber den verminderten 
 Luftdruck. Beit. z. Wiss. Bot. Funfstuck. 4: 93-148. 1900 
 
 23. Schimper, A. F. W., Pflanzen und Ameisen. 1888. 
 
 24. Schoorl, N., Zur. jodometrischen Zuckerbestimmung mittels Feh- 
 Img'scher Losung. Zeit fur. Angew. Chem. 27: 633-535. 1899 
 
 25. .Sladen, F. L., Secretion of nectar. The Beekeeper's Review 27" 
 
 419. Nov. 1914. 
 
 26. Von Rysselberghe, F., Influence de la temperature sur la perme- 
 abihte du protoplasms vivant pour I'eau et les substances dUs- 
 soutes. Recueil de 1' Inst. Bot. de Bruxelles. 5: 209-49. 1901 
 
 ' ^^- 1°?: ^i^^*.^' ^■' Ueber die Zusammensetzung einiger Nektar-Arten, 
 Zeitsch. fur Physiol. Chem. 227: 93. 1878 10 
 
 28. Wilson, A. S., Chem. News. 38: 93 1878 10 
 
 29. Wilson W. P., The cause of the excretion of water on the sur- 
 V^22 °l^r ''^- ^^^^''^- aus dem Bot. Ins. 11 Tubingen I: