CHAPTER VIII 

 ADAPTATION TO LIGHT 



189. The relation of organs to light. Light stimuU call forth 

 functional responses which produce changes in form or structure, 

 or in both. The latter is the rule, since changes in structure 

 really lead to changes in form, as will be seen later. As would he 

 expected, the leaf undergoes by far the greatest modification, and 

 distinctions between sun and shade plants are based almost wholly 

 upon it. The stem shows more or less modification in response 

 to light, owing to the fact that it usually contains chloroplasts, 

 as well as to the fact that it bears the leaves. The root is with- 

 drawn from the action of light and naturally shows only indirect 

 effects, such as might result from differences in growth, etc. The 

 loss of the leaf or even of the stem or root may occur in very diffuse 

 light or in darkness. This is more directly connected, however, 

 with the assumption of a parasitic or saprophytic habit. 



190. Influence of the chloroplasts. The clue to the effect of 

 light upon the form and stmcture of leaves is found in the position 

 of the chloroplasts. It has already been pointed out that the 

 arrangement of the latter varies with the intensity of the light. 

 Sunlight causes the chloroplasts to form rows in line -with the 

 light rays, while diffuse light leads them to take a position at 

 right angles to the ray. The three principles upon which (ho 

 structural response of the leaf to light rests are: (1) the number 

 of chloroplasts increases with the intensity of the light; (2) in 

 diffuse light, i.e., in the shade, chloroplasts arrange themselves in 

 such a way as to increase the number exposed to light; (3) in 

 sunny habitats, chloroplasts place themselves so that they decrease 

 the exposure and the consequent transpiration. 



The chloroplasts lie in close contact with the layer of proto- 

 plasm which lines the cell wall. The latter is itself clastic and 



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