196 



AMERICAN BEE JOURNAL 



June 



nies thus treated becoming weak by 

 winter or spring losses and being rob- 

 bed, because of the exposed honey. 

 Bees naturally place their brood be- 

 tween the entrance and their stores, 

 and would hasten to remove honey 

 placed in an exposed situation as prac- 

 ticed by Mr. Atwater. — Editor.] 



Pollination of Plants 



BY L. H. PAMMEL. 



I BECAME interested in bees when 

 I was a boy on a Wisconsin farm 

 in the late seventies, and shortly 

 afterwards it was my pleasure to be 

 a student in the University of Wiscon- 

 sin, where I became acquainted with 

 Dr. Wm. Trelease, the professor of 

 botany, who had published some papers 

 on the pollination of plants. It was 

 during the spring of my sophomore 

 year, when a number of students and I 

 interested in botany took a course of 

 lectures and laboratory work on this 

 subject. This opened up to me a 

 new world. I had more than once 

 heard the name of Darwin and the 

 great biological work he was doing 

 mentioned in derision. To the average 

 laymen he was known only for the 

 work he had published on the origin of 

 species and the descent of man. This 

 was a new field to me. Here was a 

 great naturalist who found "poetry" in 

 flowers, who saw and described won- 

 derful contrivances in plants to secure 

 pollination. I became acquainted with 

 the works of Hermann Mueller, Fritz 

 Mueller, Hildebrand, Asa Gray, Spren- 

 gel and Sir John Lubbock and many 

 other botanists who were students of 

 flower fertilization as it was then called, 

 later going by the name of pollination 

 and now as flower ecology. 



The subject was fascinating to me be- 

 yond measure. Not only did I become 

 acquainted with some of our wild 

 plants, but the insects important in 

 pollination. For several years I studied 

 and made observations on plants at 

 various times. My interest in the sub- 

 ject has never ceased. 



The sensitiveness of tollinating insects 

 to color and to odor. It is believed com- 

 monly that odors and bright colors in 

 flowers are of great importance as in- 

 dicators (or "signals") to insects of 

 the presence of nectar or pollen, and 

 some observers even go so far as to 

 suppose that these features have arisen 

 through natural selection, the insects 

 preferring the more fragrant and 

 showy flowers, while others go unpol- 

 linated, so that the plants bearing them 

 have no progeny. There is no evidence 

 whatever for the selection theory of 

 the prevalence of showiness and odor, 

 and even the theory that insects are 

 attracted by color and by fragrance 

 rests too little on experiment and too 

 much on the untenable assumption that 

 theory must be true, because nobody 

 knows any other role for the floral 

 features. It is a tenable hypothesis 

 that such features are without value 

 to the flowers possessing them, and 

 the "signal " theory deserves support 

 only as it is proven experimentally. 



It is not certain that insect attraction 

 is the only possible role of colored 

 corollas; it has been suggested that 

 they may play an important part in the 



chemistry of fruit maturation. Pig- 

 mented plastids may be important in 

 food making, and pigmented cell sap 

 may indicate the formation of useless 

 by-products. It is to be noted that 

 some wind-pollinated flowers are very 

 showy, as in the larch and the red 

 maple. Corollas also are of some im- 

 portance as protective organs for the 

 pollen and stigmas, especially in flow- 

 ers whose corollas close at night and 

 in stormy weather. 



The possession of a keen sense of 

 smell by pollinating insects is undoubt 

 ed, inconspicuous fragrant flowers be- 

 ing visited much more than are showy 

 ordorless flowers. The readiness with 

 which flies are drawn to sources of 

 nauseous odors is well known, and 

 they frequent ill-smelling flowers in a 

 similar fashion. Hawk moths detect 

 at a distance of several meters the pres- 

 ence of fragrant but invisible nocturnal 

 flowers, and bees have been seen to fly 

 directly toward honey artificially hid- 

 den. Indeed, there are reasons for be- 

 lieving that many insects are able to 

 detect odors that are inappreciable to 

 human nostrils. 



The possession of a keen sense of 

 color is much less certain. The only 

 insects in which color perception has 

 been definitely demonstrated are the 

 honeybees. These highly organized 

 insects often have been seen to visit 

 gaudy but nectarless artificial flowers, 

 and sometimes they attempt to get at 

 showy natural flowers that are under 

 glass. Frequently they visit colored, 

 unopened buds and wilted flowers, the 

 latter being at times approached, even 

 after they have fallen to the ground. 

 Apiarists rather generally believe that 

 honeybees are able to perceive color 

 differences, and hence they sometimes 

 paint their hives in different colors, so 

 as to aid the bees in recognizing their 

 abode. To the extent that color is 

 perceived by insects, it is a much more 

 reliable "signal "than odor, since the 

 latter often is affected by the wind or 

 masked by other odors. Probably the 

 characteristic forms of flowers serve 

 as indices to nectar, especially in the 

 case of flowers that are conspicuous 

 by their shape or by their size; some 

 observers think that form is even more 

 important than color as an insect 

 " signal." 



Some investigators believe that hon- 

 eybees not only perceive colors, but 

 that they have marked color prefer- 

 ences. Experiments with honey on 

 colored papers seem to show that bees 

 tend to visit a particular color, even if 

 others are more conveniently situated, 

 and elaborate theories have been 

 worked out on the assumption that 

 bees dislike yellow and prefer blue, 

 whence it seems to some observers an 

 easy postulate that the day of yellow 

 flowers is waning, and that of blue 

 flowers is in the ascendant. Such con- 

 clusions certainly are unwarranted. 

 The constancy of the honeybee to a 

 a given color, such as blue, does not 

 mean a preference for blue as such, 

 but the association of nectar or pollen 

 with that color. If a bee commences 

 its activities on a red flower, or on 

 honey placed on red paper, it is con- 

 stant to red. 



" In visiting flowers, bees are constant 

 not only to color, but also to form, 

 flying from flower to flower of the same 

 species. This constancy to a given 



plant species for a certain period is of 

 great advantage to the plant, since it 

 means a minimum waste of pollen. It 

 is equally of advantage to the bees, 

 since the nectar or pollen is all of the 

 same quality, and since time and energy 

 are saved in that exactly the same pro- 

 cess is repeated in each flower that is 

 visited. The collapse of the color pref- 

 erence theory is well shown in those 

 cases in which different individuals of 

 a given plant species have flowers of 

 different colors. In such species bees 

 soon learn the essential likeness of the 

 differently colored flowers, going from 

 one color to another indifferently. In 

 other words, bees learn to ignore dif- 

 ferences in color that are unaccom- 

 panied by differences in nectar or pol- 

 len. Even if bees prove to be the only 

 insects with a color sense, other insects 

 certainly are able to appreciate differ- 

 ences in tone, as they appear in a pho- 

 tographic print where whites and vari- 

 ous colors come into sharp contrast 

 with the darkness of the foliage. Simi- 

 larly, the prevalent whiteness of noc- 

 turnal flowers makes them more con- 

 spicuous than would any pigment 

 color." 



A survey of the whole subject may 

 be obtained from the English transla- 

 tion of " Knuth Handbook of Flower 

 Pollination," three volumes published 

 by the Clarendon Press, Oxford, in 

 1906. This admirable treatise has a 

 splendid summary of the more impor- 

 tant work done along the lines of polli- 

 nation up to the year 1906. Some work 

 has, of course, been done since by en- 

 tomologists and botanists. In this 

 country John H. Lovell and Grae- 

 nicher have made a number of impor- 

 tant contributions. The flower ecolo- 

 gists have lately missed the contribu- 

 tions formerly made by Charles Robert- 

 son, of Carlinville, III. Mr. Robertson 

 greatly enriched the American litera- 

 ture of the subject. 



The following agents are important 

 in the pollination of plants: I. Water 

 (Hydrophilous), Fresh Water Eel grass ; 

 II. Wind (Anemophilous), corn, wheat, 

 rye, pine, oak; III. Animal (Zoidi- 

 ophilous), birds (Ornithopilous),trump- 

 et creeper, snails (Malacophilous),duck- 

 weed, aroids, insects i^Eryiitomophilous'), 

 clover, plum, strawberry, etc. Large 

 bee flowers {Melittop/iilotis'), sage, small 

 bee flowers {Micromellittop/u'lons), pars- 

 nip, goldenrod, dogwood, small fly 

 flowers {Micromyiophilous), birthwort 

 with a temporary prison, carrion fly 

 ?[ovitr{Sapromyiophilous)ca.rr'\or\ flower 

 beetle flowers (^Cantharopiiitous), many 

 compositae, magnolia, butterfly flowers 

 (Psycliopfiidous)), pink Sphinx flowers 

 (Sphingophilous), flowers pollinated by 

 hawk moths and moths (Xoctuids). 



Loew classified flowers and the wild 

 insects adapted to them into: 1. Allo- 

 tropous. 2. Hemitropous. 3. Eutro- 

 pous. The allotropous flowers are 

 adapted to various kinds of insects 

 with a short proboscis. The hemitro- 

 pous flowers are visited by insects 

 with medium proboscis. The Eutro- 

 pous flowers are exclusively adapted to 

 insects possessing a long proboscis. 

 These flowers are therefore exclusive. 

 These flowers are pollinated by the 

 bumblebees, honeybees, and the Lepi- 

 doptera, e. g., butterflies and moths. 



Plants are either (1) self-pollinated 

 (Autogamous,) e.g., the closed flowers 

 of violet or (2) cross-pollinated with 



