12 



THE FACTORS 



[Part I 



Comparative culture-experiments have proved that plants liable to 

 desiccation have their epidermis constructed in such a manner as to 

 reduce transpiration. Protective mechanisms, such as a considerable 

 thickening of the outer wall of the epidermis, which is also well cutinized, 

 stomata sunk in pit-like or groove-shaped depressions, air-containing 

 tomentum, mere traces of which appear in cultures under dry conditions, 

 attain a high degree of development in typical xerophytes, and occur quite 

 generally under the most diverse physical conditions. Most xerophytes, 



Fig. 13. Xerophilous structure. Dry 

 climate. Mesembryanthemum Forskalii, 

 a leaf-succulent of the Egyptian desert. 

 Transverse section of leaf. After Volkens. 



Fig. 14. Xerophilous structure. Wet saline 

 soil of tropical shores. Sesuvium Portulacastrum. 

 Succulent plant. Transverse section of leaf. 



also, have the property of closing their stomata when they begin to wilt, 

 and thus of considerably depressing their transpiration ; this is, however, 

 not quite universal. Undoubtedly, under direct insolation, the pro- 

 tection afforded by the closing of stomata is much less than is often 

 imagined. 



The organs which serve for absorption in xerophytes are no less well 

 adapted for their purpose than are those for transpiration. A very rich 

 root-system distinguishes the majority of them, and many species, 

 especially epiphytes, possess a highly effective absorbing apparatus, which 

 will be described further on. 



