NUTRITION. 83 



the peculiar combinations necessary for forming the or- 

 ganic compounds ; and, secondly, some moving power 

 peculiar to life, in all probability of the nature of attrac- 

 tion and repulsion, by which the particles of that matter 

 must be moved in the directions necessary for these 

 combinations, and that such powers do exist, researches 

 into other phenomena of organized bodies seem fully 

 to prove. In regard to the regular ascent of the sap at 

 certain periods, as in spring, light and heat may be 

 looked upon as awakening stimuli to the vital powers ; 

 but independently of this there seems to be no reason 

 to deny (and some facts prove it) that it has its origin 

 to a certain extent in a tendency to a periodicity of ac- 

 tion, the exciting causes of which are to be sought for in 

 the plant itself. 



5 1 . The sap, in the course of its transmission from 

 the root to the leaves, suffers some change ; part of 

 the water, nitrogen, carbon, and soluble material, are 

 deposited ; whilst a preparation for these and other 

 elements to enter into new combinations is generally 

 going on. 



52. Digestion. On its arrival at the leaves more par- 

 ticular changes are seen, the fundamental ones of which 

 appear to be the superfluous water is given off from the 

 leaves by exhalation, carbonic acid is decomposed, the 

 oxygen is parted with, whilst the carbon is retained ; 

 besides this, however, certain other processes take place 

 in order to render the digestion of plants complete. 

 Water, at certain periods, and in many plants nitrogen, 

 is inhaled by the leaves. 



53. The decomposition of carbonic acid in the leaves 

 during the process of digestion, is one of the most im- 

 portant points to remember, and also that this decom- 

 position only takes place under the influence of light. 



54. Though evolution of oxygen only takes place 

 under the influence of solar light, when the plants are 

 growing naturally, only a very modified degree of it is 

 essential', this evolution of oxygen from the leaves 

 of plants during the day-time, must not be looked 



