MOVEMENT OF MATERIALS IN THE PLANT 139 



Although leaves removed from the plant evaporate much more water than do 

 apparently similar attached leaves, nevertheless this experiment shows that con- 

 siderably more solar energy disappears in the process of transpiration than in 

 the decomposition of carbon dioxide. 



The humidity of the surrounding air is a second condition markedly influenc- 

 ing the rate of transpiration. The less water vapor the air contains, the more 

 rapid is transpiration, and the transpiration rate decreases as the water-vapor 

 content of the air increases. l 



Temperature also influences transpiration, but the relation here is compli- 

 cated by the fact that life-processes in general are greatly affected by tempera- 

 ture. 



Movement of the air also increases transpiration. Finally, the chemical 

 properties of the soil exert a marked influence upon the amount of water evapo- 

 rated from leaves. Experiments with water cultures show that transpiration 

 is controlled both by the concentration of the solution and by the presence or 

 absence of certain substances. Thus, acids may accelerate, while alkalies may 

 retard transpiration. Addition of a small amount of some salt to distilled 

 water in which plants are rooted produces an increased rate of transpiration, 

 but addition of larger amounts causes a gradual decrease in the rate. The 

 transpiration of plants grown in solution containing the essential mineral 

 elements becomes less as the concentration of the solution is increased.'" 



Besides the external factors mentioned above, there are also internal condi- 

 tions that control transpiration, these being related to the organization of the 

 plant. First, the age of the plant is important. During the period of greatest 

 activity of the leaf, while it is still growing, the rate of transpiration is highest. 

 The reason for this is that the epidermis of young leaves is very permeable to 

 water; transpiration decreases later, but a second maximum is reached when the 

 stomata begin to function. Thereafter the rate of transpiration gradually 

 decreases as the epidermis hardens, in spite of the influence of the stomata. 



The rate of transpirational water loss from leaves is also correlated with 

 the form and character of their anatomical structures (e.g., number of stomata, 

 thickness or permeability of the epidermis, etc.). A discussion of the resistance 

 offered by plants to transpiration will be presented later, in Part II, Chapter III. 



Liquid water, as well as water vapor, is given out from many plants, through 

 hydathodes.' 1 The exudation of liquid water may partly replace transpiration, 



1 The best study of the influence of air humidity as such is: Darwin, F., On a method of 

 studying transpiration. Proc. Roy. Soc. London B87 : 269-280. 1914. Reviewed by Liv- 

 ingston in: Plant world 17: 216-219. 1914. — Ed. 



m On the influence of chemicals upon transpiration see: Reed, Howard S., The effect of 

 certain chemical agents upon the transpiration and growth of wheat seedlings. Bot. gaz. 49 : 

 81-109. 1910. On the influence of the osmotic concentration of the medium see: Briggs and 

 Shantz, 1913 [see note i, p. 137]; Tottingham, 1914, [see note d, p. 84]; Shive, 1915, 2 [see 

 note a, p. 83]; Trelease, 1920, [see note c, p. 86]. — Ed. 



" Moll, J. W., Ueber Tropfenausscheidung und Injection bei Blattern. Bot. Zeitg. 38 : 

 49-54. 1S80. Idem, Untersuchungen liber Tropfenausscheidung und Injection von Blat- 

 tern. Verslag. en Meded. K. Akad. Wettensch. Naturk. Amsterdam 2 R., 15: 237-337. 



