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EVOLUTION 



May, 1932 



visiting the more pleasant ones, had actually shaped and 

 colored and given odors to the blossoms that catered to their 

 preferences. But this would be only a guess. Until many 

 naturalists had studied thousands of species of plants for 

 almost a century, they were not well-enough informed to 

 establish a reliable theory of how the insects made the flowers. 

 But sixty years ago they had acquired so much knowledge of 

 the relations of plants to insects that they felt pretty confident 

 of the theory — which Darwin first elaborated. Since then all 

 added knowledge of botany confirms Darwin's theory, and no 

 knowledge has run counter to it. Today every botanist as- 

 sumes that flowers were developed, in the course of millions 

 of years', by the adaptations that plants made for inviting 

 and employing and rewarding the insects. 



In this brief article we can look at only a few examples of 

 those thousands of devices which have been evolved by plants 

 in their efforts to use insects as pollen-carriers. 



A series of five facts should be clear in a reader's mind if 

 he is to realize the meaning of the devices. (1) The sweet 

 juice of flowers, the nectar, is the material from which honey 

 is made; and honey is the only food of bees in a state of 

 nature; honey is for them a matter of life and death. They 

 drink the nectar, convert it into honey, and store it in the 

 comb to support life through the winter. (2) Pollen is the 

 source of the "bee bread" on which the young bees are fed. 

 (3) The whole duty of worker bees — to which they devote 

 incessant labor so long as they live — is to bring nectar and 

 pollen to the combs. (4) The whole anatomy of a worker is 

 an apparatus for extracting, carrying and converting the food 

 that is found in flowers. The mechanisms and instincts of a 

 bee are all directed to one end: making successful visits to 

 flowers. (5) All insects that depend on flowers for a living 

 are, like the bees, engaged in the most serious business of 

 their lives when they visit blossoms: if they do not secure 

 food, they die. 



And it is equally true of all those flowering plants which 

 depend on insects for pollination that they will die if they 

 do not persuade the insects to visit them. Flowers and insects 

 are engaged every minute of their lives in an unrelenting 

 struggle to exist. If we find them adapted to each other's 

 needs, we can be sure that the adaptation is not a chance and 

 not a joke; it must be a result that has evolved in the course 

 of the long ages of fierce competition to survive. 



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"One Way Passage" of bee through lady-slipper flower. 



Think of a flower whose stamens "wither before the pistils 

 are ripe for pollen. It can never fertilize itself. The species 

 would die next year if the blossoms did not succeed in entic- 

 ing bees to come to them. A bee has been visiting blossoms 

 in which the stamens have skilfully deposited pollen on her 

 head; she comes to these waiting pistils that are surrounded 

 by dead stamens; she rubs off pollen on them; the flowers are 

 fertilized. No botanist can conceive that a flower has had 

 this remarkable adaptation from the beginning of time; it 

 must have evolved at some period in the history of the 

 species. And there is no way to imagine the evolution except 

 to suppose that all flowers which varied toward this arrange- 

 ment were more likely to have descendants that would con- 

 tinue to vary still further in that direction. We can only 

 imagine that the species was thus transformed by adjusting 

 itself to the tastes of insects. 



Look at a flower which does not open imtil nightfall. It 

 has no gay colors, for they would not be visible; it has the 

 light yellow color that is most prominent in the starlight; it 

 has a strong odor; its nectar is at the bottom of a long tube. 

 It could never have been developed by its need of bees or 

 wasps. Its color, time of opening, odor, position of nectar 

 — all are adjustments to invite a certain kind of night-flying 

 moth. All its structure and habits have evolved as a response 

 to what this moth desires. 



There are many plants that have developed ways of fenc- 

 ing out unwelcome ants from the store of nectar — barriers of 

 bristly or tanglefoot hairs. Yet these flowers are so con- 

 structed as to admit the long proboscis of the bee. They have 

 been shaped in a complicated and accurate way by the attacks 

 of enemies and the visits of friends. 



In some flowers the petals form a cap over the stamens, 

 and this cap is so delicately adjusted that it is thrust aside 

 by the landing of a bee. Instantly the released stamens fly 

 up, strike the bee in the chest, and dust her with pollen, 

 which will fertilize the next flower she visits. Another similar 

 arrangement is a set of stamens that fly up and strike when 

 their base is touched. One of the set of stamens in another 

 flower is provided with a trigger, so set that when the bee's 

 proboscis stretches down for nectar it strikes the trigger and 

 sets all the stamens to vibrating. 



When you see an orchid in a florist's window, you are look- 

 ing at one of the family of plants that have gone furthest 

 in inventing machinery to work tricks on bees. One of them 

 actually provides a pool of water, on the brink of which is 

 some food that bees are eager to gnaw — so eager that they 

 often push each other into the water! When their wings are 

 draggled, they can leave the flower only by a tunnel near 

 the overflow spout. In this tunnel the pollen is stuck to their 

 backs, and so will be carried to another flower and fertilize it. 



In some of our American swamps there is a delicate little 

 flower called "sundew," because on the surface of its leaves 

 there are drops of sparkling white. These drops are so sticky 

 that they can hold on to the feet of insects; the hairs on the 

 surface of the leaf then fold over one by one and strangle the 

 insect; the plant digests its' victim. It has evolved a trap and 

 digestive fluids that enable it to eat meat. Many other plants, 

 of very different kinds, have developed other methods of 

 catching and eating insects. For example, the butterwort — 

 which has to rely on insects for carrying its pollen — captures 

 other insects by snapping leaf-edges over them while their 

 feet are caught in a viscous fluid. 



