THE LOSS OF WATER BY THE PLANT 



179 



case of water shortage, stomata of the upper leaves remain open 

 longer. As this is a characteristic feature of many xerophytes, this 

 structure has been called "xeromorphism." 



These differences in structure of the upper leaves may be due 

 to the fact that they develop under the conditions of a somewhat 

 lower water supply, which may result in smaller dimensions of their 

 cells. The same xeromorphic structures may be induced by the 

 direct influence of external factors on the plant as a whole, such 

 as increased dryness of the air, reduced soil moisture, and periodical 

 wilting. Plants developing under these conditions are smaller in 

 size, but have a greater drouth resistance. 



Fig. 79. — Vascular network in a lower (left) and an upper (right) tobacco leaf 



(after Zalensky). 



Similar changes are produced by increased light intensity. 

 Generally, sun leaves differ from shade leaves by the same charac- 

 teristics as the upper leaves differ from the lower ones. Especially 

 marked are the changes in the development of palisade parenchyma. 

 Of the many physiological characteristics of sun leaves, an in- 

 creased assimilation, at least in strong light, and an intensive trans- 

 piration are especially marked. Consequently, sun leaves may be 

 considered as xeromorphic leaves. 



The study of the anatomical and physiological peculiarities of 

 xeromorphic structures is of great importance in the determina- 

 tion of differences in drouth resistance among varieties of crop 

 plants. Recent investigations (Maximov, H. Walter) have shown 

 that resistance to drouth is not due to a reduced expenditure of 

 water, but to the capacity to endure wilting, which inevitably 



