138 PHYSIOLOGY OF NUTRITION 



Leaves removed from the plant lose much more water than those still 

 attached to the plant. Krutizky 1 found that a single leaf of Cyssus antarcticus 

 lost 10.6 cc. of water in one day, while a branch of the same plant with six 

 leaves, lost only 10.8 cc. Results obtained from studies with cut leaves are thus 

 not to be applied directly to entire plants. 



After the foregoing introductory remarks, the influence of external conditions 

 upon the rate of transpiration will now be considered. 



Light exerts a pronounced influence upon the amount of water evaporated. 2 

 For instance, three maize seedlings weighing 1.6 g. transpired in one day, 198 

 mg. in sunlight, 68 mg. in diffuse light and 27 mg. in darkness. Plants trans- 

 pire much more actively in light than in darkness. If they are transferred 

 from darkness to light, or the reverse, the rate of transpiration is not suddenly 

 increased or decreased, but the change in rate takes place gradually. 



The daily periodicity of transpiration also depends upon light. 3 The amount 

 of water absorbed during the whole period of twenty-four hours is practically 

 equal to that lost by transpiration in the same period, but there is no such agree- 

 ment between the rates of absorption and transpiration for the various hours of 

 the day; plants are generally nearly saturated with water at night but during the 

 day there is a saturation deficit/" 



All rays of the spectrum are not equally effective in promoting transpiration 

 from green plants, the maximum effect is produced in the blue and violet regions. 

 The red rays between the Fraunhofer lines B and C are next, in order of their 

 influence. The same wave-lengths of light that are most absorbed by chloro- 

 phyll are thus also most effective in promoting transpiration. 



Of all the external factors influencing transpiration, light is undoubtedly the 

 most important. The question arises as to how much light is used in this proc- 

 ess. An experiment 4 showed that sunflower leaves transpired on a sunny 

 day 275 cc. per square meter of leaf surface per hour. To evaporate this 

 amount of water requires 166,800 gram-calories of heat per hour. This leaf area 

 received 600,000 calories per hour, so that it appears that 27.5 per cent, of the 

 total amount of radiant energy received was used up in transpiration; as will be 

 remembered, only about 0.5 per cent, is used up in the assimilation of carbon. 



1 Famintsyn, A., Exchange of materials and transformation of energy in plants. [Russian.] Zapiski 

 Akad. Sci. St. Petersburg 46, Appendix, xvi + 816 p. 1883. 



2 Baranetsky, J., Ueber den Einfluss einiger Bedingungen auf die Transpiration der Pflanzen. Bot. Zeitg. 

 30: 65-73, 8ib-8ob, 97-109. 1872. Wiesner, 1877. [See note 2, p. 135-] Kohl, 1886. [See note 3. 

 P. 135] 



3 Eberdt, O., Die Transpiration der Pflazen und ihre Abhangigkeit von ausseren Bedingungen. Mar- 

 burg, 1889. 



4 Brown and Escombe, 1900. [See note 1, p. 34.] 



* Renner, O., Beitrage zur Physik der Transpiration. Flora 100 : 451-547. 1910. Idem 

 Versuche zur Mechanik der Wasserversorgung. 1. Der Druck in den Leitungsbahnen von 

 Freilandpflanzen. Ber. Deutsch. Bot. Ges. 30: 576-580. 1912. Idem, same title. 2. 

 Ueber Wurzeltatigkeit. Ibid. 30: 642-648. 191 2. Livingston, B. E., and Brown, W. H., 

 Relation of the daily march of transpiration to variations in the water content of foliage leaves. 

 Bot. gaz. 53 : 309-330. 191 2. Lloyd, F. E., Leaf water and stomatal movement in Gossy- 

 pium and a method of direct visual observation of stomata in situ. Bull. Torrey Bot. Club. 

 40: 1-26. 1913. Shreve, Edith B., The daily march of transpiration in a desert perennial. 

 Carnegie Inst. Wash. Pub. 194. 1914. — Ed. 



