CH. Ill, 7] ADJUSTMENTS TO LIGHT 53 



kinds the chlorenchyma varies in thickness from .09 to .58 

 millimeter, with a mean at .179, and hence a conventional 

 constant at .2 millimeter (Fig. 24). This variation, though 

 considerable in itself, is yet -wholly insignificant in comparison 

 with the variation in the sizes and forms of leaves, with which 

 indeed it bears no relation. Leaves of evergreen or leathery 

 type which seem specially thick, as in Rubber Plant, have no 



thicker chlorenchyma, but only a 



thicker epidermis, while the swollen 



and succulent leaves of Century Plant 



or Houseleek really combine the func- 

 tion of storage with that of food for- 



mation, and hence fall into another FIG. 24. The actual 

 category. What then determines this *^MS 

 singularly uniform thickness (or thin- in cross section, 

 ness) of all foliage leaves? The spec- th e j pp f s r ' 

 troscope, the instrument by which light the lower, one of the 

 can be analyzed with great precision, ^^^21^ 



shows that the red and blue- violet average of many kinds. 



rays of the sunlight, effective in photo- J^ 

 synthesis, are wholly absorbed by a ment on a very large 

 layer of chlorophyll, as dense as that 

 in the chloroplastids, a fraction of a 

 millimeter thick. Accordingly the ordinary chlorenchyma 

 can perform its function only when spread out in layers much 

 less than a millimeter thick. If the chlorophyll is less dense, 

 i.e. if there are fewer granules in the tissue, the effective light 

 can go deeper, and the green tissue is thicker though paler, 

 as in young stems. Furthermore, a stronger light can pene- 

 trate deeper, and hence effectively illuminate a thicker layer, 

 than a weak light ; and it is a fact that the thicker foliage 

 leaves are those which live exposed to the brightest sun, while 

 the thinner kinds occur on shaded undergrowth plants. 



Second of the adjustments is the existence of the stem, of 

 which the wide-branching structure carries the leaves aloft 

 and spaces them out in the light ; and this, as will later ap- 



