172 GENERAL BOTANY 



of these (or any) descriptions is almost useless. Each 

 point mentioned must be made out in the actual speci- 

 men, and since those described here are all common 

 plants, there should be no difficulty in this. 



Homology of thorns 



ZIZYPHUS JUJUBA, Lamk, the Jujube 



Or on the hills any other species of ZIZYPHUS. 



This very common and cultivated tree, grows chiefly 

 in dry places. The young branches and flowers are 

 covered with a thick reddish brown tomentum or layer 

 of short branched hairs set closely together, which 

 protects them from drying in a hot day. It is thickest 

 on the youngest parts and buds, where the protection 

 is most needed (fig. 37). 



At the base of each leaf are sometimes one, some- 

 times two thorns. Their position for they occur 

 nowhere else points to their being modified stipules, 

 and towards the end of the youngest branches, they 

 may be found still quite soft like ordinary stipules. 



These thorns prevent animals grazing on the branches, 

 and so afford the plant the same sort of protection. We 

 must not, however, from this, jump to the conclusion 

 that the stipules have developed as thorns in order to 

 keep animals off. The change has probably been due 

 to the dryness of the air of the places where ZIZYPHUS 

 usually grows, the keeping off of grazing animals being 



probably only an accidental result. 



8'BK' 10 

 CAPPARIS HORRIDA, Linn. 



A small tree which also grows in dry places. 

 It is protected from hot winds and from grazing 



