Botany: The Science of Plant Life

by Norman Taylor

A Giant Herb of the Banana Family, Musaceæ. It is the travelers’ tree (Ravenala Madagascariensis), the sheathing leafstalks of which hold considerable quantities of water—hence the name. Grown throughout the tropical world. (Courtesy of Brooklyn Botanic Garden.)

can go straight to the honey glands which are at the base of the flower, without touching the anthers. But their filaments are broadened out at the base, so much so that their edges touch. The honey glands are so placed that the insect must touch the broad bases of at least two filaments, between which, in fact, it must force its proboscis in order to reach the honey. The moment any particular pair of filaments are irritated by the bee, two pollen-dusted stamens fly out from their position among the petals and the anthers strike the bee with a sharp blow. Many observations prove that almost never does the bee go on with his honey sucking after this rude interruption, which has resulted in at least its head being dusted with pollen. The low, sticky stigma is so placed that it is one of the first things the bee’s head strikes as it reaches the center of the flower. Because of the position of the stamens, while they are undisturbed, it is impossible that pollen from them could have been brushed off at the bee’s entrance of the flower. And by an almost miraculous adjustment of the power of the blow by the irritated stamens, this drives off the intruder only after he has brushed his pollen-laden head over the stigma. His head at this stage is, of course, covered only with foreign pollen gathered elsewhere, but just as soon as the bee tries to get what he came for, sometimes even before he gets his reward, out fly the pair of stamens, thoroughly dusting the bee, and seeing to it that the blow is just sufficient to drive off the pollen-laden insect. No device to secure cross-fertilization could be more effective. If the blow of the stamen were only ever such a slight fraction less than it is, the bee would only stop a moment and then go on honey sucking; which, because of the release of the pollen by even the gentlest blow, would result in self-fertilization by the aid of the bee, rather than cross-fertilization. Very few, if any, bees will stand this gentle reminder to go, however, and it is a little curious that such intelligent creatures as they are supposed to be, should not realize that it is all the clever but quite harmless trick of an apparently still more intelligent flower to secure fertilization from any pollen but its own.

To attract insects and then repel them seems a little like using them as some flirts notoriously use men, only to throw them over when they are no longer interesting. In the large-flowered magnolia tree from the southeastern United States, insects, however, fare somewhat better than this. In this magnolia, which has flowers several inches long, self-fertilization is impossible as the stigmas are ready to receive a mate several days before the laggard stamens are provided with the wherewithal. Without some insect or other outside help there would be only a childless old age for this particular tree. The flower opens rather early in the season, while the nights are still cool, and as a protection from the cold, rose beetles habitually fly into them. They find a pleasant shelter under the three inner petals which arch over the honey-coated stigma, and form a snug little chamber so much warmer than the outer air that its heat is appreciable to the touch. The rose beetles, once they are inside this warm shelter, cannot get out and are often held for a few days. Then, as the stigma passes its period and the stamens are furnished with pollen, the chamber opens by the gradual withering of the petals. But the insects, in their efforts to get out, have raised a perfect dust storm of pollen with which they are naturally covered. Just as soon as they are released they are free to seek another warm shelter where the process is repeated. Thus they always enter the flower with foreign pollen, use it up impregnating the waiting ovule, and are held until the flower’s own pollen gives the signal for their release properly dusted for a renewal of the work. The premature timing of the stigma, the tardiness of the anther in producing pollen, the generation of heat and secretion of honey which frequently covers the whole stigma—there could scarcely be a better equipment for securing cross-fertilization. And without it the magnolia would be simply sterile.

There are some other flowers that hold visiting insects in a trap until cross-fertilization has been completed, and all of them by no means furnish their visitors such a snug little heated chamber as the magnolia. One vine from the eastern United States, known as the Dutchman’s-pipe, or sometimes as the pipevine ([Figure 71]), is singularly ruthless in this respect. Its flowers are of such evil odor that only carrion-loving insects, such as certain kinds of flies and gnats, ever visit them. The flower is of very peculiar structure, being formed of a hollow tube bent from its stalk first downward, and then upward. The upper part ends at the opening which is provided with a three-lobed lip or doorway. Through this the insects crawl, and they finally reach the bottom of the curved part of the flower. Behind and above them is the entrance through which they have just come. And above them, in the other curved part of the flower, is the stigma. As in the magnolia, this matures several days before the pollen from its surrounding stamens is ripe, so that self-fertilization is never possible. What is now the

plight of the insect at the bottom of the upward-pointing tubes, one leading to the organs of reproduction, the other to the exit? By an almost diabolical cunning the inside of this flower is so smooth that no insect can crawl up its slippery sides. It takes some time for the prisoner to find this out, and, in the meantime, it has explored every nook and corner of the flower by flying. In the course of this exploration it reaches and covers the stigma with pollen, for as we shall see presently, it always comes into the flower pollen-laden. Evidently becoming panicky about getting out, the insect then flies with very considerable force in every direction. Toward the true exit it naturally flies the most, and by a refinement of cruelty this is the lighter end of the flower, and therefore the obvious mode of escape for it. But the three lips and the entrance they cover are not flat across the flower, as the cover of a lunch box would normally be, but turned at such an angle as the lunch box would be if set on end. The insect, in flying toward the light, invariably hits the smooth surface just inside the three lips and falls to the bottom of the flower because there is nothing rough enough for it to cling to. Throughout most of the day, and sometimes for several days, the insect will keep up this ceaseless struggle to escape, flying first up one tube and then the other. These frantic efforts would end in exhaustion if kept up too long, and before that happens, but after the impregnated ovules have no further use for the flies, the anthers give forth plentiful supplies of pollen. Of course, the insect can scarcely avoid becoming covered with this, and then, but only then, the flower begins to wither and up its now wrinkled sides the pollen-laden prisoner can at last crawl to liberty.