CHAPTER 14 



Water 



Water is closely involved with many activities of the plant, especially 

 photosynthesis and transpiration. It fills the cell vacuoles and constitutes 

 the bulk of protoplasm. It maintains the turgidity of the tissues and thus 

 is an important factor in growth. Botanists still are very far from explain- 

 ing the complex problems of the water relations of plants. These have 

 been discussed in an extensive physiological literature (see Crafts, Cur- 

 rier, and Stocking, 1949; Kramer, 1945, 1955; Meyer, 1938; and Walter, 

 1955). 



Xeromorphy. Water is also of significance in problems of plant struc- 

 ture and thus for morphogenesis. Where it is relatively scarce or the 

 amount that can be absorbed is limited for other reasons (as in saline 

 soils) or where evaporation is excessive, plants display characteristic 

 structural features. Such xerophytes tend to have reduced leaf surfaces, 

 heavy cuticle, small and thick-walled cells, high stomatal frequency, 

 abundant mechanical tissue, and large root systems, and they often are 

 spiny or succulent. These traits, collectively termed xeromorphy, have 

 been regarded as adaptations which increase absorption or reduce tran- 

 spiration and thus maintain a sufficient water supply under dry condi- 

 tions. Xerophytes may show other adaptations such as hairy surfaces, 

 rolled leaf blades, and stomata sunken in pits or otherwise protected 

 against undue exposure to evaporation. The characteristic structures of 

 xerophytic plants have long attracted the interest of ecologists and pro- 

 vide much of the subject matter for the science of ecological anatomy. 



Such traits presumably have arisen through the action of natural se- 

 lection and are thus not ultimately attributable to the direct effect of the 

 environment. Many plants, however, if grown under conditions where 

 water is scarce or transpiration high, have been observed to assume 

 some degree of xeromorphism. Leaf surfaces will tend to be somewhat 

 reduced, cells smaller and thicker-walled, and mechanical tissue more 

 abundant. Such structural changes are clearly the result of an environ- 

 mental factor— a reduction in amount of available water. What is in- 

 herited here is this specific response to the environment. 



There has been considerable controversy, however, as to whether or 



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