THE OUTER TISSUES OF PLANTS. 259 



Imagine that across this thin, green, semi-transparent struc- 

 ture, there are from ten to thirty divisions, which prove to be 

 the successive nodes of an axis. Imagine that along the 

 edges of this leaf-shaped aggregate of internodes, there arise 

 axillary buds, some of which unfold into flowers, and others 

 of which shoot up vertically into growths like the one which 

 bears 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 Muhlenbeckia. 

 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 in Phaenogams 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 physiological 

 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 



