DIV. I 



3IORPHOLOGY 



169 



involved in arrangements to diminish transpiration are referred to 

 collectively as the xerophytic structure (XEROMORPHY). Desert 

 plants, the plants of dry rocks and many epiphytes, are naturally 

 extreme xerophytes (cf. p. 183). 



It is, however, a striking fact that xerophytic structure is also met 

 with in plants of quite different modes of life, where it is not at first 

 sight comprehensible, e.g. in plants of high mountains or of high 

 latitudes, in many swamp plants, in plants of the sea-coast (HALO- 

 PHYTES) (Fig. 195), even when, as in the case of the Mangrove 

 vegetation of tropical coasts, they grow directly in the water, and 

 lastly in many trees of the tropical rain-forest. Though much is still 

 obscure regardipg this, it is safe to assume that the majority of these 

 plants are, at least 

 periodically, in danger 

 of losing more water 

 by transpiration than 

 they can make good by 

 absorption from the 

 soil. When they occur 

 in relatively moist soils 

 these appear to be more 

 or less physiologically 

 dry for the plants, i.e., 

 to be such as to render 

 the absorption of water 

 difficult. 



Both morphological FlQ igo.Transverse section of the epidermis of Aloe nigricans. 

 and anatomical arrange- i, Inner, uncutinised thickening layer, (x 240. After 



ments are concerned STRASBURGER.) 

 in diminishing trans- 

 piration. Some of these adaptations may at the same time be pro- 

 tective against strong insolation or overheating. 



The following are anatomical features which serve to diminish 

 transpiration : thick epidermal cell walls and cuticle ; formation of 

 waxy and resinous coatings, and, in the case of stems and roots, 

 layers of cork ; reduction in the number of stomata ; narrowing of the 

 stomata and their occlusion by resin ; sinking of the stomata below 

 the general level of the epidermis, either singly (Fig. 190) or in 

 numbers in special flask-shaped depressions of the under side of the 

 leaf (e.g. Oleander), or the over-arching of the stomata by adjoining 

 cells so that they come to be situated in cavities protected from the 

 wind. Hairs, whether woolly, stellate, or scaly, which early become 

 filled with air and give the plants a whitish or grey appearance 

 (Edelweiss, Australian Proteaceae, Olive), may serve as a protection 

 against the sun's rays. On the other hand, evergreen leaves may be 

 small, leathery, and relatively poor in sap (e.g. sclerophyllous evergreen 



