DEFENSIVE EQUIPMENT OF PLANTS 141 
can be removed, more or less easily, from the plant. The 
prickles of the rose and brambleare structures of thisnature. 
The prickles of thistles are indurated, sharply-pointed 
hairs, set over all 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 this 
Fig. 49.—Stem or Fic.50.—FauseAcactra’ Fic. 51. — Stinerna 
BaRBERRY WITH (Robinia pseudacacia) Harr or NErrir. 
Lear-SPrnes. WITH SPINY STIPULES, (HicHiy Macnirtep.) 
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 
has enabled them to spread over fields and pastures. 
(f) 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. 
