144 



THE STORY OF THE PLANTS. 



ite, from which the others are descended with 



various modifications. 



To this central type belong a large number of 



well-known plants, both useful and ornamental, 



though more particularly deleterious. Among 



them may be mentioned 

 the various thistles, 

 such as the common 

 thistle, the milk thistle, 

 the Scotch thistle, and 

 so forth, most of which 

 have their involucres, 

 and often their leaves 

 as well, extremely 

 prickly, so as to ward 

 off the attacks of goats 

 and cattle. The bur- 

 dock, the artichoke, the 

 saw - wort, and the 

 globe-thistle also be- 

 long to the same cen- 

 tral division. Among 

 these earlier compos- 

 ites, however, there is 

 one group, that of the 

 centauries, which leads 

 us gradually on to the 

 next division. Our com- 

 monest centaury in 



purple florets, all equal and Britam (knOWn tO boyS 



similar. as hardheads) has all 



the florets equal and 

 similar, and looks in the flower very much like a 

 thistle. But one of its forms, and most of the 

 cultivated garden centauries, have the outer florets 

 much larger and more broadly open than the cen- 



FiG. 36. — Flower-head of a thistle, 

 consisting of very numerous 



