42 



BRITISH PLANTS 



lignified walls, non-living, and containing only air. 

 The cells are cemented together in sheets and columns, 

 and form very effective screens between the living cells 

 filled with water and the external air (Fig. 11). A 

 covering of cork on stems and shoots serves a similar pur- 

 pose. The presence of sclerenchyma in large and long-lived 



leaves gives them mechan- 

 ical support, and keeps 

 them from being easily 

 torn and injured — e.g., 

 New Zealand flax (Phorm- 

 ium tenax), Aspidistra. 



6. Some xerophytes con- 

 tain oil in their tissues, 

 especially in the leaves. 

 Where present, it un- 

 doubtedly serves to check 

 evaporation. Many strand 

 and semi-desert plants are 

 quite remarkable for their 

 fragrance — e.g., rosemary, 

 bay - laurel, sage, worm- 

 wood, etc. 



Xerophytic Forms of Leaf 

 and Shoot. 



The most important are : 

 1. The Needle -Type, as 

 in the pme. The leaf is 

 evergreen, thick and tough, 

 with a much reduced sur- 

 face ; the internal cells are 

 packed closely together, 

 the cuticle is thick, and 

 the stomata are reduced in 

 number and sunk in pits. 

 2. The Concrescent Type, as in many cypresses and 

 jumpers (Fig. 6). The leaves are thick and evergreen 

 very small, erect, and fused with the stem along nearly 

 their whole length. There is very little internal air- 

 space, the cuticle is thick, and the surface smooth and 

 polished. 



Fio. 10. — Salicornia herbacea (Glass- 

 wokt), sHowiNO Succulent Stems 



AND Mtt^uTE AdpBESSED LeaVES. 



(Slightly Reduced. Aftek 



So WERE Y.) 



