26 MISC. PUBLICATION 257, U. S. DBPT. OF AGRICULTURE 



The broad-leaved trees are thus seen to transpire twice as much as 

 the conifers in January and 1.6 times as much in March. 



ENVIRONMENTAL FACTORS 



Light — Sun and Shade Leaves 



Many studies have been made on the relation of light to transpira- 

 tion, and for some years these studies seemed quite contradictory; 

 but within the last few years the contradictions have been ironed out 

 and considerable order is being developed from the previous chaos. 

 Light, first of all, tends to produce a thickening of the leaves (i. e., 

 a xeromorphic condition), especially in regions where water is lacking, 

 this interrelation between the light and the water supply being one of 

 the principal reasons for the contradictions in the literature. 



Some authors found that light caused a checking of transpiration, 

 while others reported an increase, and not until it was found that the 

 result obtained depended largely upon the amount of available water 

 was it possible to reconcile conflicting results. Measurements of 

 transpiration in sun and shade leaves may also differ, depending upon 

 whether the transpiration is related to the dry weight of a leaf or to the 

 leaf area. If a beech leaf grown in the shade seems to evaporate more 

 per unit of dry weight than one grown in the sun, this may be due to 

 the more delicate nature of the former as compared with the tougher 

 character of the latter. Hence it is quite possible for the transpira- 

 tion per unit of weight of sun leaves to be less and yet for the total 

 amount of water transpired by them to be more, as pointed out by 

 Biisgen and Munch (33, p. 228). Light thus affects transpiration 

 either (1) indirectly, by influencing the structure of the leaves, or (2) 

 directly, through its effect upon the water loss itself. 



The effect of light in changing the structure of the leaf has been 

 reported by many authors. Grosglik (76) showed that the mesophyll 

 of Eucalyptus globulus Labill. was thicker, especially the palisade tissue, 

 in the presence of strong light. Geneau (68) likewise showed that the 

 palisade layer of shade leaves in Salix rosmarinvfolia Hort. was 0.06 

 mm thick while that of the sun leaves was 0.15 mm. In Quercus 

 pedunculata, the corresponding figures were about 0.04 mm and 0.09 

 mm. Stahl (202) showed that the sun leaves of Fagus were 3 times 

 as thick as the shade leaves, while Geneau reported that the sun leaves 

 of Taxus baccata L. were about 1.4 times as thick as the shade leaves. 

 In Q. pedunculata the sun leaves were 1.7 times as thick as the shade 

 leaves, while the sun leaves of Fagus sylvatica were twice as thick as the 

 shade leaves. Stahl also showed that the cells of the mesophyll were 

 more compact in sun leaves so that the intercellular space was con- 

 siderably smaller. In Sambucus he calculated that the volume of the 

 intercellular space was 16 percent of the leaf volume in sun leaves and 

 26 percent in the shade leaves; in Fagus these figures were 19 and 29 

 percent, respectively. 



Nordhausen (163) thought that these differences were not the direct 

 result of light but were predetermined by the conditions (light, 

 humidity, etc.) under which the buds formed the previous year. 

 Maximo v (148, p. 34?'), however, points out that these structures in 

 the developed leaves are not indirect aftereffects produced in the 

 leaf structure by the environmental conditions of the previous year 

 but are direct effects of the stem structure, which are in turn deter- 

 mined by the environmental conditions, The water supplied to the 



