Page:Popular Science Monthly Volume 30.djvu/118

106 of the Tweed this identification should be mentioned, as French newspapers remark, under all reserves.

Almost all the thistles have purple florets, and purple, it may be safely assumed, is the primitive color of the whole thistle-head tribe. Some of them, indeed, fade off gradually into pink and white; but such reversion to a still earlier ancestral hue is everywhere common and easily brought about by stress of circumstances. The thistles in the lump are composites by family, and the apparent flower is really a flower-head, containing an immense number of small, bell-shaped, five-petaled florets, with the petals united at their base into a deep tube. The honey rises high in the throat within, and is sucked chiefly by bees and burnet-moths, who form the principal fertilizers of the entire group. Purple is the favorite color of these advanced flower-haunters, and it seems probable that all the purple blossoms in nature have been evolved by their constant and long-extended selective action. Nothing can be more interesting than to watch a great burly humble-bee (one of the large black sort) bustling about from flower-head to flower-head of the pretty, drooping, welted thistle on a bright summer's day, with his proboscis constantly extended in search of food, and unconsciously carrying the pollen-grains about his head and legs from the florets of one blossom to the sensitive surface of the next in order.

After the flowers have been duly fertilized, the thistle-seeds begin to swell, and the down around them to grow dry and feathery. This down, so familiar to all of us among the autumn fields, has doubtless played no small part in the dispersal of the thistles. It is to their floating seeds (or rather, to be strictly accurate, their fruits) that the entire family owe a great part of their existing vogue and unpopularity. In almost all the composites the tiny calyx grows out into much the same silky down on the ripe fruit, but in hardly any other case save perhaps those of the dandelion and the common sow-thistle, does it form so light and airy a floating apparatus as in the true thistles. Wafted about on the wings of the wind, the thistle-down is blown easily hither and thither, alighting everywhere, far and near, and finding out fresh spots for itself to root and thrive on every side. Not only does this plan insure the proper dispersal of the seeds, however: it also provides for that most important agricultural need, the rotation of crops. Long before scientific farming had hit upon the now familiar rotatory principle, hundreds and hundreds of plants in the wild state had worked it out practically for themselves under stress of the potent modifying agency of natural selection. For thistles can no more grow on the same spot for an indefinite number of generations than corn or turnips can; they require to let the soil on which they live lie fallow for a while from time to time, or be occupied by other and less exhausting crops. Hence it follows that in nature innumerable means exist for favoring or insuring the dispersal