500 



BOTANY 



LIGHT 



A certain illumination is necessary for the assimilation of C0 2 , and 

 the adaptation to changes in the intensity of light are very obvious. 

 The contrast between two individuals of the same species exposed to 

 light of different intensity is very striking. Individuals growing in 

 shade have the leaves many times larger than the same plant exposed 

 to the full rays of the sun, and thus expose a much larger number of 

 green cells to the action of the diffuse light rays. Where the light 

 conditions are intermittent, it is possible, in many cases, to demon- 

 strate a change in the position of the chlorophyll-bodies, dependent 

 upon the change in the intensity of light. In the leaf of a Moss, for 

 example, the disk-shaped chromatophores spread themselves evenly 

 over the outer cell-wall in diffuse light, but retreat to the side- 

 walls and turn their edges 

 to the light, when it is 

 strong. 



Where the illumination 

 is very intense, as in arid 

 regions, the small size of the 

 leaves is very marked, and 

 there are sometimes tem- 

 porary provisions against 

 too intense illumination, of 

 which one of the most con- 

 spicuous is the development 

 of red pigments in the young 

 leaves of many plants, espe- 

 FIG. 468.- Ranunculus multibus L. aerial da]1 in the Tropics, but also 

 leaf; w, submersed leaf. (After GOEBEL.) , J . , 



shown by the young shoots 



of Tea-roses, and the purple and red young shoots of such decid- 

 uous trees as the Red-maple. It is supposed that in these cases 

 the red pigment acts as a screen for the protection of the young 

 chloroplasts. 



Mesophytes. The term Mesophyte has been adopted for those 

 plants which grow under average conditions of light, etc., and com- 

 prises the ordinary plants of temperate and moist tropical regions. 

 In these plants the leaves are of normal size, and usually disposed 

 so as to expose a maximum surface to the light. To this end, 

 the leaves are so arranged as to avoid excessive overlapping and 

 shading. In many cases, the leaves are placed in numerous rows 

 along the stem, and not infrequently, where the leaves are closely set, 

 the petioles of the lower ones are longer than the upper ones, so that 

 the blade of the leaf projects beyond those lying above it (Fig. 465). 

 In other cases the leaves are much divided, so as to let the light 



