Water 329 



of leaf structure. Miiller-Stoll (1947b) has evidence that xeromorphy of 

 plants in peat bogs is due to a deficiency of nitrogen rather than of water. 

 He fertilized such plants and observed a marked increase in leaf area and 

 cell size and a decrease in stomatal frequency, cell- wall thickness, and 

 venation. Lack of nitrogen and lack of water thus seem to produce 

 similar structural changes. These two factors are associated in other 

 morphogenetic phenomena. 



Experimental Work. Many experiments have been performed to de- 

 termine directly the effect on plant structure of varying amounts of 

 water in the soil or in the air. Only a few of these can be mentioned 



here. 



Rippel (1919) studied white mustard growing in moist and in dry 

 soil. Vein length per unit area of leaf surface was considerably greater 

 in dry soil. In both, it increased progressively from the first leaf to the 

 fifth, and this gradient was steeper in dry soil than in wet. 



Penfound (1931) paid particular attention to stem anatomy and found 

 that, although increased soil moisture reduced xeromorphic traits, it in- 

 creased the relative amount of xylem in the stem. 



Cain and Potzger (1940) brought Gaijlussacia plants into the green- 

 house. They varied the amount of available water and also grew some of 

 the plants in front of a fan. Though dry air and fan induced some 

 xeromorphy, the mesophyll was considerably thicker in plants in the 

 moister soil, contrary to most earlier observations. 



Simonis (1952) studied four genera grown in soils of high and low 

 water content. In all, the leaf surface was reduced under water deficit, 

 but the morphological responses of different plants were somewhat dif- 

 ferent. Water content tended to be unchanged under dry conditions, 

 or sometimes even was increased, and succulence was generally greater. 

 Simons (1956) grew year-old apple seedlings in greenhouse pot cul- 

 ture in moist and dry soil. Reduction in water supply affected leaf area 

 and also thickness and size of cells in epidermis, palisade, and xylem. 



Fewer experiments have been done on the morphogenetic effects of 

 differences in the humidity of the air. Eberhardt (1903) grew a wide 

 variety of plants under bell jars, maintaining light and temperature the 

 same in all but changing the humidity. Dry air tended to produce the 

 ordinary xeromorphic traits and also an increase in hairiness ( Figs. 14-3 

 and 14-4). The results of Lebedincev (1927) and Rettig (1929) were 

 much the same. The effects of dry air were more pronounced when the 

 soil was also dry. 



At the opposite extreme from xeromorphy, produced by water deficit, 

 are those changes that result from submersion in water. As a rule, the 

 roots of such plants are small or lacking, the vascular and mechanical 

 tissues poorly developed, the leaves thin and often much dissected, the 



