326 Morpho genetic Factors 



without relating them to water conservation but simply as direct or 

 indirect results of decreasing cell size with progressively higher leaf 

 insertion. Small cell size, in turn, grows out of the greater difficulty 

 with which water is obtained by the higher leaves, since they have to 

 lift it farther and against the competition of the lower ones. It follows 

 that at the critical period of rapid leaf growth, which results primarily 

 from cell expansion through the absorption of water, the cells of the 

 upper leaves cannot attain the size of the lower ones. The other struc- 

 tural traits are a consequence of this basic difference. That such a con- 

 clusion is correct is indicated by other evidence, such as the fact that if 

 lower leaves are removed while the upper ones are still growing the 

 latter will resemble in structure leaves that would normally be lower 

 on the stem. 



Xeromorphy in these upper leaves therefore seems unlikely to be an 

 adaptation for reducing water loss. Indeed, it was later shown that 

 upper leaves may transpire more rapidly than the lower ones. These 

 results have cast doubt on the adaptive character of the traits of 

 xerophytes in general. Maximov calls attention to the fact that when 

 water is abundant many xerophytes transpire more rapidly than meso- 

 phytes and it is only under drought conditions that their water loss is 

 markedly cut. He attributes this, and therefore the quality of drought 

 resistance in general, not to any structural traits but to protoplasmic 

 characters, notably osmotic concentration and changes in cell colloids 

 that would enable the plant to conserve its water supply and thus 

 endure dry conditions better than other plants. Eckardt (1953) agrees 

 with this conclusion. The physiology of drought resistance has been re- 

 viewed by Iljin (1957). 



In earlier years a number of Russian investigators, assuming that types 

 with small cells were more resistant to drought than those with large 

 cells, determined for various cereal varieties their "anatomical coefficients" 

 (chiefly cell size), hoping to find a means of identifying resistant types 

 by direct inspection. This possibility was not supported by the work of 

 Maximov. More recently, however, Lai and Mehrotra (1949), working 

 with sugar cane, have found that some cell characters, notably small 

 size, seem in certain cases to be associated with drought resistance. 



Farkas and Rajhathy ( 1955 ) reexamined anatomical gradients in some 

 herbaceous plants, particularly tomato, and found that cell size in 

 leaves decreases from below upward and that number of stomata per 

 unit area increases, thus again confirming Zalenski (Fig. 14-1). Under 

 dry conditions this gradient is much steeper. They found several other 

 physiological gradients some of which may be explained as dependent 

 on that for water. In others the relation to the latter is not clear. Stage 

 of development of the leaves also complicates the problem here. 



