The Water Relations of Leaves 



79 



E. S. Clements 



Fig. 44. Vertical sections of 

 leaves of Hippuris, a water 

 plant. The upper figure 

 shows an aerial leaf, the lower 

 figure a submerged leaf. The 

 aerial leaf is much thinner 

 and the tissues more compact. 



stomata. Light affects the opening of stomata and raises the 

 temperature of the leaf, and consequently increases the rate of 

 transpiration. 



Intense light and drought decrease the 

 size and number of leaves and increase 

 the compactness of the mesophyll, the 

 amount of cutin, mucilages, hairiness, and 

 mechanical tissue. Incidentally, these 

 changes retard transpiration. Hence 

 these external factors, by producing 

 changes in the physical and chemical 

 processes within the plant, indirectly 

 modify transpiration. 



The higher the temperature is, the 

 greater the rate of transpiration, not only ^^ ^^^ ^^^^^^^^^ ^^^^ ^^^ 

 because the water in the mesophyll cells guard ceils form but the sto- 

 changes to vapor more rapidly, but be- mata do not open, 

 cause the vapor particles move out of the leaf faster. 



Submerged and floating leaves. An examination of a sub- 

 merged leaf on any pondweed shows that it has no stomatal open- 

 ings. Sometimes the guard cells are formed but do not separate 

 (Fig. 44). The floating leaves of water hlies and other pond 

 plants have stomata only on the upper surfaces. Being com- 

 pletely surrounded by water, submerged plants have no transpira- 

 tion. It is also certain that they get their carbon dioxide directly 

 from the water through the epidermis, for carbon dioxide is found 

 dissolved in pond waters, often in larger proportion than in the air. 



In water-h'ly leaves the upper surface is covered by a cuticle 

 that is not readily made wet, and it has stomata that do not open 

 until the leaf is above water, li the leaves are raised entirely 

 above the surface of the water, as sometimes happens when the 

 plants are crowded, both surfaces develop stomata. 



Desert plants and water storage. In the desert, where the air 

 is very dry and the scanty rainfall is confined to one or two periods 



