May, 1932 



EVOLUTION 



Page 



What Made the Flowers? 



By HENSHAW WARD 



TF ANYONE asks what made the leaves of plants, he is 

 proposing a question that has no answer, for no one can 

 tell how leaves happened to come into existence. But 

 botanists know that "What made the flowers?" is one of the 

 most pregnant queries ever put by a naturalist, and that the 

 answer is the prettiest one in the domain of evolution. Insects 

 made the flowers. 



Before this answer can be understood, we must know what 

 a flower is. It is an apparatus by which plants make seeds. 

 There is a bewildering variety of flowers, working in various 

 ways; but the most common kind, the kind with which this 

 article deals, consists of two organs. One of these, called a 

 stamen, produces the male element that goes to the making 

 of an embryo. At the top of the stalk of a stamen is a knob 

 in which are fine grains of pollen. These grains correspond 

 to the male element, the sperms, in animals. The second 

 organ, called a pistil, produces the female element that goes to 

 the making of an embryo. At the base of its stalk is an ovary 

 which contains eggs that are waiting to be fertilized. 



Botany for Colleges — Ganong: Macntillan 



In flower at left, Salvia pratensis, hinged lever arrangement brings 



stamens down on body of bee. Moth in Yucca flower at right, carries 



pollen from another flower, tamps it down and lays egg in it. 



If you look closely at a typical flower — say a wild rose — 

 you will see at the base of the colored petals a circle of several 

 dozen stamens, each of which is a little stalk that bears aloft 

 its pollen-sac. Within this' circle of stamens is a cluster of 

 pistils; each of them is rough and sticky on the upper end of 

 its stalk, and each one terminates below in an egg-sac. The 

 business of every stamen is to convey its pollen to the top of 

 some pistil; the purpose of every pistil is to capture a pollen- 

 grain that will fertilize its egg. Unless the egg is fertilized 

 by pollen, it is barren and cannot produce a seed. This 

 process of fertilizing is called "pollination." And the need 

 of pollination is the origin of all the colored flowers. 



If you think of an ear of corn, you can realize what pol 

 lination means. On the cob of the young ear were clustered 

 a dozen rows of pistils waiting to be fertilized. From each pistil 

 extended a long, delicate thread, which reached to the end 

 of the cob and stretched out into the air. We call this set of 

 pistils the "silk" of the ear. Two feet above the ear. at the 

 top of the cornstalk, was a cluster of stamens, which we call 

 the "tassel." When mating-time comes for the plant, the 

 pollen-sacs of the tassel open and spill their contents. The 

 microscopical grains float down through the air. When one 

 of them lights on the tip of a thread of silk, it is straightway 

 transformed into a thing of life. It sends out a most slender 



and delicate tube, so small that it can force a passage down 

 through the core of the silk thread, dissolving, really eating, 

 its way along the whole length until it reaches the egg. There 

 its nucleus combines with the nucleus of the waiting egg. And 

 what then? At that moment the embryo of a new kernel of 

 corn is created. This grows as the weeks pass until it be- 

 comes one of the hundreds of juicy kernels that are set on 

 the surface of the cob. If it is not gobbled by hungry animals, 

 but is allowed to grow hard, is kept through the winter, and 

 put under ground next spring, it will sprout and grow into 

 a new cornstalk. 



Thus we see that a corn plant is complete within itself: its 

 ovaries are fertilized by its own pollen. If one cornstalk stood 

 all alone in the midst of a large space, it could be self- 

 sufEcient with its own pistils and stamens. Yet in an ordinary 

 cornfield, on a windy day when the tassels are ripe, the air is 

 full of pollen, and many silky pistils are fertilized by pollen 

 that is blown from another plant. Such pollination is called 

 "cross-fertilization." Some plants cannot be fertilized by their 

 own pollen; and experiment has shown that most plants are 

 more surely and richly fertilized by the pollen from another 

 plant. The general rule of nature is that most plants re- 

 quire, or are better off with, cross-fertilization. Grasses and 

 trees send out in the spring vast numbers of pollen-grains, 

 which fill the air in uncounted millions. The pine, for ex- 

 ample, commits to the breezes a kind of pollen-grain that is 

 wafted by means of air-sacs on either side of it. Perhaps only 

 one in a million finds a pistil. It is a fearfully extravagant 

 method, but it succeeds. It accomplishes the cross-fertilization 

 that plants are in need of. 



Plants with conspicuous flowers are not so extravagant. 

 They have found a cleverer way of conveying pollen. They 

 lure and bribe insects to do the carrying. Watch this bee that 

 is buzzing up to a head of clover. She has been attracted by 

 the bright red spot in the landscape, for she knows that it 

 advertises food. She likes the odor, because it certifies to the 

 presence of food. She confidently runs her tongue into the 

 bell of one of the flowerets, and finds, sure enough, a sip of 

 the nectar that she can convert into honey. That is all she 

 thinks of — nectar. But the flower has quite other thoughts. 

 It quickly sticks some pollen-grains on to the bee's head. The 

 bee flies to another flower. As she presses eagerly forward 

 for the next drop of nectar, some of the pollen-grains that 

 she has been transporting are rubbed off and gathered in bv 

 the rough, sticky tops of the greedy pistils. The bee has. all 

 unconsciously, accomplished cross-fertilization for the clover. 



If botanists knew only of the ways in which insects carry 

 pollen for clover and roses and buttercups and lilies, they 

 would never suppose that they knew the answer to "What 

 made the flowers?" They might guess that these four flowers 

 had developed, through a long course of evolution, such 

 showy and sweet-scented mechanisms as were adjusted to the 

 taste of insects, that the blossoms had been gradually shaped 

 by the success of those plants that varied in such ways as to 

 grow more and more attractive to insects. It might be a likely 

 guess that insects, by avoiding the less pleasant flowers and 



