246 PHYSIOLOGICAL DEVELOPMENT. 



this greater productioa of chloropliyll next to the uppoj 

 surface of a leaf, is directly consequent on the greater 

 amount of light received. Here, as in so many other cases, 

 we must regard the difi'erentiation as in part due to direct 

 equilibration and in part to indirect equilibration. Fa- 

 miliar facts compel us to conclude that from the begiuniug, 

 each individual foliar organ has undergone a certain im- 

 mediate adaptation of its surfaces to the incidence of light ; 

 that when there arose a mode of growth which exposed the 

 leaves of successive generations in similar ways, this im- 

 mediately-produced adaptation, ever tending to be transmitted, 

 was furthered by the survival of individuals inheriting it in 

 the greatest degree ; and that so there was gradually esta- 

 blished that diffeience between the two surfaces which each 

 leaf displays before it unfolds to the light, but which becomes 

 more marked when it has unfulded.* 



From the ordinary cases let us now pass to the exceptional 

 cases. We will look first at those in which the two faces of 

 the leaves differ but little, or not at all — their circumstances 

 being similar or equal. Leaves that grow in approximately- 

 upright attitudes, and attitudes which do not maintain the 

 I'elative positions of the two surfaces with constancy, may be 

 expected to display an unusual likeness between the two 

 surfaces ; and among them we see it. The Grasses may be 

 named as a group exemplifying this relation ; and if, instead 

 of comparing them as a group with other groups, we compare 



* The current doctrine that chlorophyll is {he special substance concerned 

 in vegetal assimilation, either as an agent or as an incidental product, must 

 be taken with considerable qualification. Besides the fact that among the 

 A Igcc there are many red and black kinds which thrive ; and besides the fact 

 that among the lower Acrogens there are species whicli are purple or chocolate- 

 coloured ; there is the fact that Phsenogams are not all green. We have the 

 CopinT-Beech, we have the black-purple Coleus VersohaffMii, and we have the 

 retl variety of Cabbage, whicli seems to flourish as well as the other varieties. 

 Chlorophyll, then, must be regarded simply as the most general of tho colour- 

 ing matters found in those parts of plants in which assimilation is beuig effected 

 by the agency of li^ht. 



