THE LOSS OF WATER BY THE PLANT 



165 



intercellular spaces, and the diffusion of the vapor thus formed 

 through the stomata. The laws governing diffusion through stomata 

 have been discussed previously, and it has been shown that the 

 great number of these very small openings in the cuticle strongly 

 favors diffusion. This process may go on through the stomatal 

 openings, constituting only 1 to 2 per cent of the leaf area, almost 

 at the same rate as if the cuticle did not exist and the internal cavi- 

 ties were fully open to the air. A 

 diagram illustrating this is shown in 

 Fig. 70. Comparative determinations 

 of the intensity of transpiration from 

 a leaf and of evaporation from a free 

 water surface have shown that this 

 condition may be realized. In some 

 very vigorously evaporating plants, 

 the relative transpiration may attain 

 the magnitude of 0.8 to 0.9; while 

 under favorable conditions 0.4 to 0.5 

 are average figures. If transpiration 

 were proportional to the area of the 

 openings, then the values would not 

 exceed 0.01 to 0.02. Thus we see 

 that, per stoma, the diffusion rate of 

 water vapor is veiy high. 



The diffusion capacity of the 

 epidermis supplied with innumerable 

 stomata is so high that the plant seldom uses it to full capa- 

 city. This is true at least for the process of transpiration. The 

 stomata play also an important role in assimilation, as they are the 

 openings giving access to carbon dioxide. Actually, transpiration 

 is always considerably lower than its calculated value. Where- 

 from, the conclusion may be drawn that besides changes in the 

 stomatal openings, other factors may exert a checking influence on 

 transpiration. These factors will be discussed later. 



53. The Stomatal Apparatus of the Plant. Methods of 

 Studying Stomatal Movements. — One of the most important 

 peculiarities of the stomata is their ability to change the diameter 

 of the opening. Light and the water content of the leaf are the 

 chief external factors responsible for this. In the majority of 

 plants, the stomata are opened in light and closed in darkness. 



Fig. 70. — Diagrams showing 

 diffusion of water vapor from 

 an open vessel A and through 

 a multiperforated membrane B 

 (after H. Walter). 



