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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 mecha ni s ms, . such _as__a_considemble 

 thickening of the oute£_wall oLt he epidermis ,_which_is also well cutmized, 

 -stOTTiTitar sunl? TiTpit-like orgroov e-shaped ^depressioas^^ajr^containing^ 

 tomentum, mere-t^^aees-of-^aTicHIappear in cultures under ^ry con ditions, 

 attain^aTTigh-d^reeoTdevelopment in ^typical^^Yophytes^a^d occur quite 

 gehefattjrimderThelnbsf dIverse"^Hysiw conditions. Most xerophytes, 



Fig. 13. Xerophilous structure. D:y 

 climaie. Mesembryanthemum Forskalii, 

 a leaf-succulent of the Egyptian desert. 

 Transverse section of leaf. After Volkens, 



Fig. 14. Xerophilous structure. We/ 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 tliem, and many species, 

 especially epiphytes, possess a highly effective absorbing apparatus, which 

 will be described further on. 



