Page:Popular Science Monthly Volume 30.djvu/120

108 vast previous evolutionary history. Starting apparently from blossoms with five distinct and separate yellow petals, like the buttercups, the ancestors of thistlehood gradually progressed, as it seems, by insect selection, to a condition something like that of the harebell or the Canterbury bell, in which the petals have coalesced at their bases into a single large and united tube. Clustering together next into closely serried heads, like those of the scabious, the rampions, and the common blue sheepsbit, they endeavored to make up for the individual minuteness of their dwarfed flowers by the number and mass collected in a group on the summit of each stem. In this way they gradually assumed the distinctive crowded composite form, each floret consisting of a tubular five-lobed corolla, a calyx reduced to hairs or down, and single tiny seed-like fruit. Of this stage in the development of the family, the simpler and less specialized members of the thistle group, such as the unarmed saw-worts and the Alpine saussurea, are now the best surviving representatives. From some such early central form, the evolving composites split up and diversified themselves into all their astonishing and almost incredible existing variety. Some of them, varying but little in minor details from the parent stock, acquired prickly leaves and grew into the thistle kind, or developed hooked and sticky involucres, and were known as burdocks. Others, producing at their edge a row of brilliantly colored and attractive florets, which serve the purpose of petals for the compound head, branched off into all the marvelous wealth of daisies, asters, sunflowers, marigolds, dahlias, golden-rods, ox-eyes, and cinerarias. In yet others the whole mass of the florets, central as well as external, has assumed this ray-like or strap-like form; and to this group belong the dandelions, hawk-weeds, salsifies, lettuces, sow-thistles, chiccories, nippleworts, and cat's-ears. By far the most successful of all flowering plants, the composites have taken possession in one form or another of the whole world; and among the entire wealth of their extraordinary diversity there is no group more universally fortunate than the common thistle. What from the purely agricultural point of view we describe as a very persistent and almost ineradicable weed, from the higher biological point of view we should more properly regard as a dominant and admirably adapted species of plant. The one conception is merely narrow, practical, and human; the other is positive, philosophical, and universal.—Longman's Magazine.