370 



PLANT PHYSIOLOGY 



the size of each stoma bemg smaller. Terminal leaves have 

 also a thicker network of vascular bundles, a greater number of 

 hairs per surface area, and a thicker layer of pahsade tissue. 

 This dependence of structure on position has been called ''the 

 rule of Zalensky." Figure 118, showing the vascular network 

 in an upper and lower leaf of tobacco, may serve as illustration 



of this rule. 



These anatomical pecuharities may be correlated with physi- 

 ological differences. The upper leaves are distinguished by 

 higher assimilation and more intense transpiration. The 

 osmotic pressure of their cells is higher, and in wilting the upper 



Fig. 118. — Vascular network in a lower (left) and an upper (right) tobacco leal 



{after Zalensky) . 



leaves draw water from those lower down. Thus when perma- 

 nent wilting takes place, the lower leaves will release all their 

 water and die. In case of water shortage, stomata of the upper 

 leaves remain open longer and thus may continue to assimilate 

 over a greater period. As these are characteristic features of 

 many xerophytes, this structure is called ''xeromorphism." 



These differences in the 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 structure may 

 be induced by the direct influence of external factors on the plants 

 as a whole, such as increased dryness of the air, reduced soil 

 moisture, and periodic wilting. Plants developing under 

 such conditions are smaller in size but have a greater drought 

 resistance. 



