72 Flowers and their Unbidden Guests, 



in Knaviia dipsacifolia Host, ■which is figured as an 

 example at Plate I. fig. 7. 



It may again be laid down as a general rule, that 

 the closer to the flower the greater the accumula- 

 tion of the prickly formations ; and every one must have 

 noticed that in our common thistles the lower leaves are 

 less prickly than the upper ones, and these again than 

 the leaves of the involucrum. There are many plants 

 moreover where the stem and leaves are perfectly smooth 

 and free from prickles, while the involucrum is studded 

 with rigid points. Of this latter condition good ex- 

 amples are furnished hy many species of Centaurea, 

 and in Plate I. fig. 8, I have given in illustration of 

 this an enlarged drawing of the involucrum of C. 

 cyanus, L, a widely spread species of this group. In 

 this plant neither stem nor leaves show any trace of 

 prickles, while every one of the lower and middle in- 

 volucral bracts is beset with stifi', sharp, scarious teeth 

 round its whole margin, excepting where overlapped 

 by a neighbouring bract. All these teeth curve down- 

 wards, so that their fine needle-like points are pre- 

 sented to any animal that would creep up to the 

 flowers. The involucrum thus furnishes a protection 

 which is deterrent even to ants, who are not easily 

 stopped by opposing points; and the protection is so 

 much the more complete, inasmuch" as the bracts of 

 the involucrum overlap like tiles, so that an animal 

 that has by luck got over a lower bract will find itself 



