232 PHYSIOLOGICAL DEVELOPMENT. 



surface, there are at least two la5'ers ; and the bark coating 

 the substance of a shoot, besides being itself compound, 

 includes another tissue lying between it and the wood. What 

 is the physical interpretation of these facts ? 



When a mass of what we distinguish as inert matter is 

 exposed to external agencies capable of working changes in 

 it — when it is chemically acted upon, or when, being dry, it 

 is allowed to soak, or when, being wet, it is allowed to dry — 

 tlie changes set up progress in an equable way from the 

 Bui-face towards the centre. At any time during the process 

 (supposing no other action supervenes) the modification 

 wrought, first completed at the outside, either gradually 

 diminishes as we approach the centre, or ceases suddenly 

 at a certain distance from the centre. But now suppose that 

 the mass, instead of being inert, is the seat of active changes 

 — suppose that it is a portion of complex colloidal substance, 

 permeable by light and by fluids capable of affecting its 

 unstable molecules — suppose that its interior is a source of 

 forces continually liberated and diffusing themselves out- 

 wards. Is it not likely that while at the centre the action 

 of the internallj'-liberated forces will dominate, and while at 

 the surface the action of the environing forces will dominate, 

 there will be between the two a certain place at which their 

 actions balance ? And may we not expect that this will be 

 the place where the most unstable matter exists — the place 

 outside of which the matter becomes relatively stable in the 

 face of external forces, and inside of which the matter be- 

 comes relatively stable in the face of internal forces ? 



Be this or be this not the explanation, the well-known fact 

 is that the inner wall of each vegetal cell is a delicate mem- 

 brane, the primordial utricle, composed of that nitrogenous 

 substance specially characterized by instability ; and that 

 outside of this is the cellulose layer, and inside of it the 

 granular colouring matter. And the similarly well-known 

 fact is, that in each phsenogamic axis the cambium layer, 

 which shows its relative instability by the activity of the 



