RELATION OF LEAF STRUCTURE TO PHYSICAL FACTORS 21 



of aeration, that of keeping food material on the shortest possible 

 -route to the bundles. 



Vesque & Viet (81) carried on a year's experimentation with 

 plants in the laboratory and out of doors, with particular reference 

 to the effects of humidity and light. They grew plants under bell- 

 jars in different combinations of the two factors, and concluded that 

 humidity has the same effect as darkness on plants, perhaps by 

 decreasing the transpiration stream. 



Pick (82) studied the histology of a number of sun and shade 

 leaves and the stems of plants poor in foliage. He concluded that 

 in all cases of sun and shade leaves with the same extent of surface, 

 the former are thicker than the latter, and furthermore, that growth 

 in all directions is decreased by weaker light, but that the air-spaces 

 of shade leaves are larger in most cases. He inclined to the opinion 

 that the elongated form of the typical palisade cell is ancestral, and 

 that it is either further developed on exposure to the sun, or hindered 

 by the shade. He has concluded this from the fact that plants with 

 a typical development of palisade cells show already a sharply de- 

 fined elongation of the hypodermal cell-layer in the younger leaves 

 of the bud-layer. Moreover he found that there is no significant dif- 

 ference in the young leaves of sun and shade forms of the same 

 species. On the other hand there are also leaves which do not de- 

 velop the elongated cells until maturity. In the larger number of 

 plants growing on sunny spots, however, the palisade in the upper 

 part of the leaf is ancestral, but for its typical development the in- 

 fluence of strong light intensities is necessary. Deviations from this 

 rule are to be found in the vertically placed leaves of some plants. 



Stahl (83) drew the following conclusions from his investiga- 

 tions concerning the influence of light on the finer structure of 

 leaves. Many plants are capable to a high degree of adapting the 

 structure of their leaves to various degrees of light. Sun-leaves 

 are, as a rule, smaller, tougher and thicker than shade-leaves. In 

 sun-leaves the palisade reaches a high degree of development, while 

 in shade-leaves it is reduced and even lost altogether, or takes the 

 form of " funnel-cells." Shade leaves possess loose sponge with 

 unusually large air-spaces which are considerably reduced in sun 

 leaves. The elongated cell form in which the chloroplasts take up 

 the " profile " position is the one best adapted to strong intensities 

 of light, whereas the flat sponge cell is best fitted to weak light. 

 Opposed to " plastic " forms are stable ones which show the same 

 leaf structure under all conditions of light ; this is especially true of 

 3 



