DROUGHT RESISTANCE 301 



of soil, hence the aerial portions of the plant may receive a fairly adequate 

 supply of water even when the rainfall is scanty. 



Similarly, many drought resistant species are characterized by relatively 

 small leaves, hence the total area of foliage exposed may be small in propor- 

 tion to the absorbing capacity of the root system. The leaves of others abscise 

 with the advent of the dry season, and their transpiring surface is thus rela- 

 tively small during the period of greatest stress upon the hydrostatic sj'stem. 



The structural peculiarities of xeromorphic leaves such as thick cuticle 

 and hypodermal sclerenchyma are such as to greatly retard cuticular trans- 

 piration. Since during drought periods the stomates of xerophytes are closed 

 most or all of the time, the low rate of cuticular transpiration aids in the 

 conservation of the water remaining in the plant. 



Formerly it was believed that drought resistant species were characterized 

 by low transpiration rates, and that they are able to withstand drought 

 conditions largely because of their economical expenditure of water. Inves- 

 tigations by Maximov (1929) and others have shown clearly that the trans- 

 piration rates of most such species are as great as those of typical mesophytes, 

 whenever the soil water supply is adequate. The frequently observed low 

 transpiration rates of xerophytes are due, not to any inherent peculiarities of 

 structure or physiological behavior, but to the fact that the water content of 

 the soil in which they are rooted is so low that little or no absorption can 

 occur. 



Another misconception, long current, was that xerophytes are more effi- 

 cient in reducing the soil water content than mesophytes. The previous 

 discussion of the effect of the type of plant upon the wilting percentage of 

 a soil indicates that there is no valid evidence for such a belief (Chap. XVI). 



As the prior discussion has indicated, during a prolonged period of soil 

 water deficiency the store of water in a plant is gradually depleted, largely 

 as a result of cuticular transpiration. This is true even of xerophytes. One 

 result of this gradual water loss is a progressive increase in the severity of the 

 stress in the internal hydrostatic sj-stem. The ensuing gradual dehydration of 

 the tissues sooner or later results in the death of species possessing relatively 

 little drought resistance. Many "drought-enduring" species, on the other hand, 

 can endure this condition, which is physiologically equivalent to permanent 

 wilting, for months at a time without suffering irrecoverable injury. 



It seems clear, therefore, that one of the basic factors in the drought 

 resistance of plants is a capacity of the cells to endure desiccation without 

 suffering any irreparable injury. According to Iljin (1930) death of plant 

 cells as a result of drying is not due primarily to the desiccation of the proto- 

 plasm, but to the destructive effects upon the protoplasm of various mechanical 



