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Page:Popular Science Monthly Volume 65.djvu/423

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419
SOME PLANTS WHICH ENTRAP INSECTS.

death is followed by decomposition, and in the absorption of the products of this the plant accomplishes the end for which it possesses the traps—it gets its needed nitrogen.

In the pitcher-plants the urns of water are set and all else is left to the curiosity of the insects, the inner structure of the leaf being such—as we shall soon see—that a secure trap-door is not needed. The half dozen species of pitcher-plant (Sarracenia) exhibit considerable variety in the form of their leaves. In the purple species, the only one growing north of Virginia, PSM V65 D423 Bladderwort trap enlarged drawing.pngFig. 2. A Longitudinal Section of a Bladderwort trap greatly enlarged, showing the Hairs, the Mouth and the Trap-door the leaves form a rosette-like cluster procumbent on the mud or moss and bent upward so as to give the mouth a horizontal position. The stalk is exceedingly short and the leaves, heavy with water, never rise above the surface of the swamps and bogs which are the only home of the plant. At blooming time a stalk is sent upward for a foot or more, bearing the curious leathery flowers which nod to one side in a manner which has led some one of vivid imagination to call this the 'side-saddle plant.' The water which fills all but the youngest of the leaves is simply rain-water which has fallen in, and has nothing to do with the water supply of the plant in the ordinary sense. In the water are pretty sure to be some struggling insects trying to float or to crawl up the sides of the pitcher, some others whose struggles are over, and the remaining legs and wings of still earlier victims. All sorts of flying and creeping things seem to have a natural curiosity to examine hollow caverns such as the pitchered leaves appear to them. Perhaps, too, they are drawn to the plant by its rich red coloration or the striking veins which mark the mouth of the pitcher; indeed, one botanist has found drops of honey arranged in a row up the side of the pitcher, a lure to guide the steps of the insect directly to the interior. The wing which runs up one side has been thought, too, to serve a use in preventing insects from crawling round and round the pitcher, and to direct their steps upward to the slippery edge of the mouth. Once falling from the edge into the water, slim indeed, is the chance that any but the most active of insects will get out. The sides of the interior are not only steep, but are exceedingly smooth and offer no foothold by which to regain the top. And even if the bedraggled creature should succeed in crawling up beyond the slippery zone it would encounter an array of long stout hairs crowded close together and pointing downward. Over this ambush none but the most long-legged of insects can crawl, and