Ch. Ill, 



AD.JUST:^1EXTS TO LIGHT 



53 



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

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

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

 coiLsiderable 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 

 tj-pe which seem specially thick, as in Rubber Plant, have no 



thicker chlorench}-ma, but only a 



thicker epidermis, while the swollen 



and succulent leaves of Century Plant 



or Houseleek really combine the fmic- 



tion of storage with that of food for- 



mation, and hence fall into another 

 category. What then determines this 

 singularly uniform thickness for thin- 

 ness) of all fohage leaves? The spec- 

 troscope, the instrument by which light 

 can be analyzed with great precision, 

 shows that the red and blue-violet 

 rays of the s unli ght, effective in photo- 

 sjTithe.sis, are wholh" absorbed by a 

 layer of chlorophyll, as dense as that 

 in the chloroplastids, a fraction of a 

 millimeter thick. AccorcHngly the ordinary chlorenchyma 

 can perform its function only when spread out in layers much 

 less than a millimeter thick. If the chlorophjdl 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 hght can pene- 

 trate deeper, and hence eilectively illuminate a thicker la^-er, 

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

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

 the thinner kinds occur on shaded undergro^rth plants. 



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

 which the -nide-branching structure carries the leaves aloft 

 and spaces them out in the hght ; and this, as -nill later ap- 



FiG. 24, — The actual 

 thickness of the chloren- 

 ch\-ma of leaves, as seen 

 in cross section. 



The upper, one of the 

 thinnest, is Abutilon : 

 the lower, one of the 

 thickest, is Pelargonium ; 

 the intermediate is the 

 average of many kinds. 



(The lines were drawn 

 accurately by measure- 

 ment on a very large 

 scale, and reduced pho- 

 tographically.) 



