Grafts et al. — 200 — Water in Plants 



of nursery stock to prevent excessive water loss during transplanting and 

 storage. Wax emulsions, because of their ease of application, are now 

 largely replacing the earlier melted wax treatment (Emerson and Hil- 

 DRETH, 1933; Miller, Nielson, and Bandemer, 1937). A technique 

 of evaluation, through use of controlled environment and infra-red pho- 

 tography, of foliar injury caused by wax and oil emulsions has been de- 

 scribed by Comar and Barr (1944). 



Physiological Effects of Water Deficits:— A condition of water 

 deficit in the plant, whether brought about by low soil moisture, high solute 

 concentration in the soil solution, or desiccating atmospheric conditions, 

 greatly modifies the transpi rational, photosynthetic, assimilatory, respira- 

 tional, and reproductive activities of the plant. A more thorough study of 

 the changes which take place in the colloidal properties of cell protoplasm 

 upon the withdrawal of water as a consequence of freezing or desiccation 

 is necessary in order to understand the physicochemical and biochemical 

 reactions that take place during drought and freezing, and to learn why in 

 some cells such changes lead to irreversible coagulation of the protoplasm 

 and death, whereas other cells are able to withstand such changes. 



Emphasis of early work on drought resistance was laid particularly on 

 an ecological consideration of xeromorphism and is adequately reviewed 

 in Maximov's work (1929a). Xeromorphic structure is characterized 

 anatomically hy 1) a decrease in dimensions of cells including guard cells, 

 2) a greater number of stomata per unit area, J) a denser network of 

 veins, 4) a thickening of the cuticle, and 5) a greater density of hair cov- 

 ering. All or part of these anatomical features may be induced or inten- 

 sified by continued mild exposure to drought conditions. Physiologically, 

 a xeromorphic condition is characterized by i) a more intense assimila- 

 tion ; the chloroplasts function only part time because of a restriction of 

 their activity during periods of water deficit, and 2) an increase in stomatal 

 transpiration but a decrease in cuticular transpiration. In spite of Sey- 

 bold's (1929) attempt to revive the earlier view of Schimper that xero- 

 phytes have a lower transpiration due to their structure, it is generally 

 agreed that this is not the case when the stomata are open and, in fact, un- 

 der these conditions xeromorphy is often characterized by more intense 

 transpiration (Maximov, 1931; Vassiliev, 1930). 



Recently Bennet-Clark (1945) has suggested that the anatomical 

 features which had formerly been considered as helping to reduce the trans- 

 piration rates of xerophytes (i.e., hairs, sunken stomata, and leaf roll) 

 actually benefit the plant, tending to prevent stomatal closure by maintain- 

 ing a higher humidity at the leaf surface. This increases the duration of 

 active photosynthesis by allowing gaseous exchange to continue. 



Current studies stress the effects of drought on biochemical reactions. 

 Maximov (1941) has reviewed the course of hydrolytic enzyme reactions 

 that take place under dry conditions. He has noted their effect on photo- 

 synthesis and has discussed theories of water-supply relations and starva- 

 tion with special reference to transpiration, respiration, and dry matter 

 production. The effects of withholding water as a means of hardening 

 plants to withstand drought is also discussed. 



Enzymatic activity is greatly affected by the water balance of the plant 

 cell. Catalase and reductase activity is high during all phases of water 

 deficit as is also the velocity of the hydrolytic activity of amylase. The 



