30 



MISC. PUBLICATION 25?, U. S. DEPT. OF AGRICULTURE 



In summing up the work on light, one may say that, everything 

 else being equal, plants transpire more in light than in its absence and 

 that transpiration increases with the intensity of light, provided there 

 is an abundant supply of moisture. When water is lacking or if the 

 site is notably dry, the transpiration of the shade leaves may exceed 

 that of the more xeromorphic sun leaves. Also, as pointed out by 

 Wiesner ("38), a plant taken from the darkness into the light shows at 

 the beginning stronger transpiration than it does later on, when it 

 finally reaches a stationary value. The same thing happens when a 

 plant is brought from the light into the darkness; the change causes 

 an original increase, which later decreases to a constant value. In 

 this latter case the stationary value is arrived at earlier than when the 

 plant is taken from the darkness into the light. 



The explanation of these phenomena is probably not simple but 

 depends on various factors. Wiesner has shown that both heat and 

 light rays increase the transpiration, and he explains the effect of 

 these rays by assuming that they increase the permeability of the 

 plasma membranes of the transpiring cells; this increase in permea- 

 bility naturally permits an easier escape of the included water. The 

 effect of light is probably associated with very complicated reactions, 

 accompanied by the transformation of the light rays into heat, but 

 for a discussion of the physics involved, the reader is again referred 

 to Seybold (189). One must not confuse the effect of light on perme- 

 ability, however, with that of water deficiency, which tends to de- 

 crease the permeability of sun leaves, as pointed out by Huber (109). 



Other less abstruse factors, however, are present. A part of the 

 increased transpiration in sun leaves can be explained by the greater 

 number of stomata (20 to 60 percent greater per unit surface) they 

 contain and by the difference in their chlorophyll content. Hessel- 

 mann (95) was among the first to call attention to the increased num- 

 ber of stomata in sun leaves in general, and his results have been 

 confirmed by other workers interested especially in trees. Table 10, 

 giving data from the work of Schramm (181), shows the difference 

 in the number of stomata per square millimeter in sun and shade 

 leaves of various woody species. 



Table 10. — Number of stomata per square millimeter in shade and sun leaves oj 



various tree species 



Species 



Stomata per 

 square millime- 

 ter in— 



Species 



Stomata per 

 square millime- 

 ter in— 





Shade 

 leaves 



Sun 

 leaves 



Shade 

 leaves 



Sun 

 leaves 





Number 

 113 

 468 

 450 

 425 

 170 



Number 

 416 

 810 

 800 

 608 

 365 





Number 

 215 

 478 

 450 

 70 

 162 



Number 

 860 







1,265 





Tilia platyphyllos Scop 



Sambucus nigra L 



759 





165 





364 









In consequence of their greater number of stomata and the fact 

 that stomatal transpiration is much greater than cuticular, sun leaves 

 transpire more than shade leaves. Shade leaves also have less need 

 for cuticular protection against evaporation, and since they are 



