262 PHYSIOLOGICAL DEVELOPMENT. 



habitually of a deeper green than the under. Microscopic 

 examination shows that this deeper green results from the 

 closer clustering of those parenchyma-cells full of chlorophyll 

 that are in some way concerned with the assimilative actions ; 

 while beneath them are more numerous intercellular passages 

 communicating with those openings or stomata through 

 which is absorbed the needful air. Now when it is remem- 

 bered that the formation of chlorophyll is clearly traceable to 

 the action of light when it is remembered that leaves are pale 

 where they are much shaded and colourless when developed 

 in the dark, as in the heart of a Cabbage when it is remem- 

 bered that succulent axes and petioles, like those of Sea-kale 

 and Celery, remain white while the light is kept from them 

 and become green when exposed; it cannot be questioned 

 that this greater production of chlorophyll next to the upper 

 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 differentiation as in part due to direct 

 equilibration and in part to indirect equilibration. Familiar 

 facts compel us to conclude that from the beginning, each 

 individual foliar organ has undergone a certain immediate 

 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 immediately- 

 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 established 

 that difference between the two surfaces which each leaf dis- 

 plays before it unfolds to the light, but which becomes more 

 marked when it has unfolded.* 



* The current doctrine that chlorophyll is the 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 

 Algce there are many red and brown kinds which thrive ; and besides the 

 fact that among the lower Archegoniates there are species which are purple or 

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

 have the Copper-Beech, we have the black-purple Colens VerschafTelfii, and 

 we have the red variety of Cabb'oge, which seetns to flourish as well as the 



