Grafts et al. 



188 



Water in Plants 



pervious coatings, thus limiting water loss to that which passes through the 

 cuticle of the upper leaf surface. Using this method and comparing trans- 

 piration from wilted leaves and leaves with the stomata sealed, Agamov 

 (1927) showed that even with wilted leaves having apparently completely- 

 closed stomata, cuticular transpiration was slightly augmented by stomatal 

 transpiration. 



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-20 



G.M.T 



Fig. so. — Diurnal fluctuation in the wettability of leaf cuticle as shown by change in contact 

 angle of water drops. The solid line with circles represents contact angle; the vertical bars show 

 significant differences for the points. The dashed line represents temperature; the light solid line, 

 relative humidity. Data of Fogg (1944). 



Under certain abnormal conditions such as wilting, or with age, the 

 cuticle of leaves may develop fine cracks through which water loss is in- 

 creased. HoLLE (1915) describes such cuticular fissures in wilted leaves 

 of Rochea falcata, and Kamp (1930) found that the cuticle of old leaves 

 split into fine fissures. Although no observation of such cuticular cracks 

 were made by Mendel (1945), he felt that such cracking of the normally 

 very slightly permeable cuticle of citrus (Bartholomew, 1931; Oppen- 

 HEiMER and Mendel, 1939) might explain an observed anomolous trans- 

 piration rate following irrigation of certain trees which had suffered from 

 low soil moisture. The transpiration rate from these trees failed to reflect 

 stomatal movement. 



That cuticular transpiration may reach appreciable values is shown by 

 the work of Rudolph (1925), who found that cuticle on the leaves of 

 many plants permits a considerable water loss. As much as 30 per cent 

 of the total water loss was found to be cuticular. In contrast, desert suc- 

 culents may survive for months and even years with no water uptake, be- 

 cause cuticular transpiration is reduced to a negligible value. 



An extensive discussion of the effects of environmental factors on the 

 rate of cuticular transpiration has been presented by Gaumann (1942) 

 with a theoretical consideration of the possible effects of the structural 



