DEFENSIVE EQUIPMENT OP PLANTS 141 



can be removed, more or less easily, from the plant. The 

 prickles of the rose and bramble are structures of this nature. 

 The prickles of thistles are indurated, sharply-pointed 

 hairs, set over aU parts of the assimilating surfaces. They 

 probably arose in the course of Natural Selection among 

 stiff-haired xerophytes, being favoured by 

 the fact that sharp points protect them 

 against animals. Besides these prickly hairs, 

 spines are also present on the leaves. In thia 



Fig. 49. — Stem of 

 Barberky with 

 Leat-Spinbs. 



Fio. 50. — False Acacia 

 (Bdbinia pseudacacia) 

 WITH Spiny Stipules. 



Fio. 51. — Stinging 

 Haib of Nettle. 

 (Highly Maonifibd. ) 



case continued growth has been checked at certain points 

 along the leaf-margins, with the result that sharp-pointed 

 spines have been produced. Thistles naturally inhabit 

 dry waste places, but their efficient protective equipment 

 itas enabled them to spread over fields and pastures. 



(/) Stinging Hairs. — These are also epidermal out- 

 growths, and they are only skin-deep — e.g , the stinging- 

 nettle (Urtica, Fig. 51). They are broken by a slight 

 touch, the sharp point enters the skin, and an acrid fluid 

 is automatically injected into the wound. 



