242 PHYSIOLOGICAL DEVELOPMENT. 



them. Imagine a whole plant thus seemingly composed of 

 jointed willow-leaves growing from one another's edges, 

 and some conception will be formed of the Coccoloba. The 

 two facts which have meaning for us here are first, that the 

 performance of leaf-functions by these axes goes along with 

 the assumption of a leaf-like translucency ; and, second, 

 that these flattened axes, retaining their upright attitudes, 

 and therefore keeping their two faces similarly conditioned, 

 have these two faces alike in colour and texture. 



That physiological differentiation of the surface which 

 arises iu Pheenogams between axial organs and foliar organs, 

 is thus traceable with tolerable clearness to those differences 

 between their conditions which integration has entailed 

 partly in the way above described, and partly in other ways 

 still to be named. By its relative position, as being shaded 

 by the leaves, the axis is less- favourably circumstanced for 

 performing those assimilative actions effected by the aid of 

 light. Further, that relatively-small ratio of surface to 

 mass in the axis, which is necessitated by its functions as a 

 support and a channel for circulation, prevents it from taking 

 in, with the same facility as the leaves, those surrounding 

 gases from which matter is to be assimilated. Both these 

 special causes, however, in common with that previously 

 assigned, fall within the general cause. And in the fact that 

 where the differential conditions do not exist, the physio- 

 logical differentiation does not arise, or is obliterated, we 

 have clear proof that it is determined by unlikenesses in the 

 relations of the parts to the environment. 



274. From this most general contrast between aerial 

 surface-tissues those of axes and those of fblia we pass now 

 to the more special contrasts of like kind existing in folia 

 themselves. Leaves present us with superficial differentia- 

 tions of structure and function ; and we have to consider tho 

 relations between these and the environing forces. 



Over the whole surface of every phaenogamic leaf, as over 



