

BOTANY OF THE LIVING PLANT 



the former especially on the dry areas of the ( >ld, the latter of the New 

 Worl • Insomi is the stem swells to an almost spherical 



term, by which humus the greatest possible proportion of bulk to 

 : t. uned. By virtue of the water stored up within its tissues 

 a , pl.mt can live for several months without any external supply 



of water. A spiny or thorny character is common in Xerophytes (Fig. 

 141 1. and is a marked feature in dry districts such as the veldt of 

 ith Africa. A consequence of this is protection against the attack 

 of herbivorous animals. Many Xerophytes possess extensive and 

 deep root-systems (e.g. Wehuitschia) ; moreover, their root-hairs are 

 characterised by high osmotic pressures and this allows them to 

 dehydrate the soil more thoroughly than is usual in Mesophytes. 



Along with these features go various other structural modifications. 

 Thickened epidermal walls and cuticle are common (Fig. 142), and this 



Fig. 142. 

 Part of a transverse section of the xerophytic leaf of Hakea, showing a stoma 

 lv depressed below the well-developed epidermis, which has greatly thickened 

 , covered by a thick, continuous cuticle. ( > 150.) F. O. B. 



together with a free development of mechanical tissue frequently gives the 

 es a leathery texture. Hairiness is common. The stomata are frequently 

 sunk in deep pits (Figs. 52, 142) in which a pocket of moist air collects and is 

 likely to impede diffusion through the pore. Hairs have a like effect on eva- 

 poration from the cuticle. Protection of stomata is also achieved in other 

 • s by the lengthwise rolling of the leaf in such a way that the stomata lie 



