Water 



327 



Shields (1950), who has reviewed the whole subject of xeromorphy, 

 agrees in general with Zalenski, Maximov, and Yapp that this type of 

 structure has little significance as an adaptation in drought resistance. 

 She also emphasizes the importance of physiological factors in relation to 

 water loss. Many of the structural characteristics of xerophytes, she sug- 

 gests, may be the result of physiological differences. Thick cell walls and 

 abundance of mechanical tissue may result from active photosynthesis 

 in a plant where all its products cannot be used in growth because of 

 the shortage of water. 



Ashby ( 1948Z? ) , however, presents evidence that the relative xeromorphy 

 of the upper leaves, at least as indicated by cell size, is not due to corn- 



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Fig. 14-1. Gradient of cell size in tomato leaves. Graph showing stomatal number (in 

 a given microscopic field ) , epidermal cell size ( in microns ) , and leaf surface ( in square 

 centimeters) for the first four leaves of five young tomato plants. These are arranged 

 in each case according to the ascending order of stomatal number for each leaf class. 

 ( From Farkas and Rajhdthy. ) 



petition for water but to the influence of immature leaves on those above 

 them, an influence which may be hormonal in character. 



Leaves on the outside, and particularly the south side, of the crown 

 of a tree (sun leaves, p. 312) are often considerably thicker and more 

 xeromorphic in appearance than those in the center (shade leaves). 

 This has been attributed to the direct action of light. This difference may 

 be due in part to water relations, for Hanson ( 1917 ) showed that, on 

 the outside of the crown, conditions favored much more rapid evaporation 

 than in the interior. He found that leaves on the outside are smaller, 

 more deeply lobed, and lower in water content and that they transpire 

 faster per unit of area (Fig. 14-2). This has frequently been confirmed. 

 Huber (1926) agrees that sun leaves result from a water deficit. Soding 



