178 TEXTBOOK OF PLANT PHYSIOLOGY 



Instead of the efficiency of transpiration a converse value is 

 often used in agricultural literature, that is, the number of grams 

 of water spent in the production of 1 g. of dry weight, which is 

 called the transpiration coefficient, or the water requirement. For 

 the majority of crop plants this value is from 300 to 400 under the 

 conditions of central Europe. It was believed for a time that the 

 adaptability of a plant for growing in drouth regions may be 

 judged from the transpiration coefficient. It seemed natural that 

 the more xerophytic a plant is, the more efficiently it will spend 

 water. The extensive investigations of Briggs and Shantz, in the 

 United States, and of Maximov and Alexandrov in the Caucasus, 

 have shown, however, that no direct correlation between drouth 

 resistance of a plant and its water requirement can be established. 

 In many typical xerophytes, for instance, the sagebrush, the effi- 

 ciency of transpiration is very low, but its intensity is high. 



58. Effects of Environment on Structure of Transpiring 

 Organs. Xeromorphism and Drouth Resistance of Plants. — 

 The transpiring organs of plants, which in practically all cases 

 are the leaves, exhibit a considerable plasticity. Leaves of dif- 

 ferent plants and even of the same plant show considerable differ- 

 ences in structure, depending on the external environmental factors 

 under which they develop. 



That certain regularity in the structure of leaves depends on 

 their position on the plant has been established by Zalensky. He 

 has found that leaves on the upper half of a stem are always differ- 

 ent from those below. The higher the position of a leaf, the smaller 

 are the dimensions of its cells, but the greater the number of sto- 

 mata per surface area, the size of each stoma being smaller, how- 

 ever. Terminal leaves have also a thicker network of vascular 

 bundles, a greater number of hairs per surface area, and a thicker 

 layer of palisade tissue. This dependence of structure on position 

 has been called the Rule of Zalensky. Figure 79, showing the vas- 

 cular network in an upper and a lower leaf of tobacco, may serve 

 as illustration of this rule. 



These anatomical peculiarities may be correlated with physio- 

 logical differences. The upper leaves are distinguished by higher 

 assimilation and more intense transpiration. The osmotic pres- 

 sure of their cells is higher, and in wilting the upper leaves draw 

 water from those lower down. When permanent wilting takes 

 place, the lower leaves will release all their water and die. In 



