178 PLANT PHYSIOLOGY AND ECOLOGY 



cotyledons growing at the edge of shaded brooks is probably a 

 contrivance to increase water loss. 



The number of stomata for an equal area of the epidermis is 

 greater in sun leaves than in shade leaves. This is generally true 

 of all sun and shade plants, but it is most clearly shown by the 

 different habitat forms, or ecads, of the same species. The sun 

 leaf of Allionia linearis has 180 stomata, and the shade leaf 90 

 stomata per square millimeter. In Scutellaria hrittonii, the 100 

 stomata per sq. mm. of the sun leaf are reduced to 40 in the shade 

 leaf. However, in a stable species such as Erigeron speciosus, 

 the number of stomata remains unchanged in those plants that 

 have moved into the shade. The presence of the larger number of 

 stomata in the sun plant, which is exposed to the greater water 

 loss, has already been explained. 



195. The form of leaves. The form of the leaf is largely deter- 

 mined by the action of light upon the chloroplasts and the conse- 

 quent change in the form of the cells that contain them. Owing 

 to the direction in which they elongate, sponge cells tend to produce 

 an extension of the leaf at right angles to the light rays. On the 

 other hand, palisade cells extend the leaf in line with the falling 

 rays. In consequence, leaves which contain an excess of sponge 

 tissue are relatively broader, while those in which palisade is 

 preponderant are relatively thicker. Since plants economize 

 material and energy in so far as possible, the broadened leaf tends 

 to be thin, and the thickened leaf to be narrow. In accordance, 

 shade leaves, i.e., those that consist largely or wholly of sponge 

 tissue, are broader and thinner, and often larger, than sun leaves 

 of the same species. Sun leaves, on the contrary, are thicker, 

 narrower, and often smaller than shade leaves. What is true of 

 the sun and shade forms of the same species holds for sun and 

 shade plants generally. 



196. Changes of outline, size, and thickness. The outline of 

 shade leaves is more nearly entire than that of those in the sun. 

 This is easily proved by comparing the sun and shade forms of 

 a species with lobed or divided leaves, though the rule is not with- 

 out exceptions. In Fig. 21 the outline of the shade form is more 

 entire in Bursa and Thalictrum, but less entire in Machceranthera. 

 Leaf prints of this kind serve more satisfactorily to illustrate the 

 increase in size and decrease in thickness produced by the increase 

 of surface in the shade leaf. In all such comparisons, however, 



