ADAPTATION TO LIGHT 177 



it is evident that palisade tissue is primarily, if not wholly, a 

 protection against water loss. Since the latter is due to light 

 or to water, palisade may be developed chiefly by the one or the 

 other according to conditions. 



Protection against water loss in consequence of sinilight is 

 brought aljout by the arrangement of the chloroplasts in vertical 

 rows so that they screen each other. Palisade cells greatly 

 decrease the transpiration due to low humidity by being so closely 

 placed that the outward movement of the moist air is hindered. 

 The air passages between them are usually reduced to the narrowest 

 of chambers, and often to mere lines. The truth of these state- 

 ments is not affected by the fact that leaves with much palisade 

 tissue, i.e., sun leaves, sometimes transpire twice as much as 

 shade leaves of the same size. This is explained by the much 

 greater activity of chloroplasts in the sunlight. As a result, sun 

 leaves require a larger supply of carbon dioxide, and the number 

 of stomata is correspondingly increased, often being doubled. 

 In consequence, the loss of water through the stomata is neces- 

 sarily increased. The small size of the air passages in palisade 

 tissue would seem to prevent the rapid movement of carbon 

 dioxide to the chloroplasts. This is apparently compensated 

 by the fact that the greater demand for this gas causes it to move 

 most rapidly toward those points where it is being used in the 

 largest quantities. 



194. Changes of the epidermis. The appearance of the chloro- 

 plasts in the epidermal cells of plants growing in diffuse light is the 

 only change directly traceable to light. Chloroplasts are regularly 

 present in the epidermal cells of woodland ferns and of submerged 

 plants. They are also found in those shade forms of sun species 

 in which the outer wall of the epidermis has become thin enough 

 to admit carbon dioxide. The absence or slight development of 

 hairs in shade plants is an advantage, because it prevents the further 

 weakening of the already diffuse light. The value of arched epi- 

 dermal cells and of epidermal papillae in controlling the absorjition 

 of light by shade plants seems to be slight. The factor that has 

 called forth these modifications and the jirimary purjwse they 

 serve must still be regarded as unsettled. The increasctl size of 

 the epidermal cells in many shade forms seems to be for the jnirpose 

 of increasing translocation and water loss, and to bear no direct 

 relation to light. The extreme size of these cells in certain mono- 



