106 THE POPULAR SCIENCE MONTHLY. 



of the Tweed this identification should he mentioned, as French news- 

 papers 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 ac- 

 tion. 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 bead 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 unpopular- 

 ity. 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 5 

 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, how- 

 ever : 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 this- 

 tles can no more grow on the same spot for an indefinite numher of 

 generations than corn or turnips can ; they require to let the soil on 

 which they live lie fallow for a wdiile from time to time, or be occu- 

 pied by other and less exhausting crops. Hence it follows that in 

 nature innumerable means exist for favoring or insuring the dispersal 



