CHAPTER XXVIII 



GENERAL REVIEW 



The living plant is in a state of unceasing activity, absorbing 

 and storing energy supplied from without, setting free and 

 dissipating it from within. The expenditure of energy may 

 be manifested in movement, or it may not be externally 

 perceptible, being employed in working the internal 

 mechanism of the body — such, for instance, as the distribu- 

 tion of water, which, as I have elsewhere shown, involves 

 a considerable expenditure of energy. The fundamental 

 importance of photosynthesis is, that it is the process by 

 which the plant absorbs the energy it requires, the radiant 

 energy of sunlight, and stores it in the form of latent or 

 potential energy in the organic products of the process. 

 The energy so stored can readily be set free again and become 

 kinetic, by the chemical decomposition of the organic sub- 

 stances, manifesting itself in heat, electric current or 

 movement. 



All these changes are effected by the living protoplasm 

 and are the expression of its physico-chemical reactions. 

 This is made clear by the observation that all the various 

 manifestations of them that have been made accessible to 

 investigation are affected in a similar manner by any given 

 stimulus or change in internal or external conditions. 



The Physiological Factor in Photosynthesis 



It is clear that the physiological factor is most impor- 

 tant, inasmuch as any protoplasmic stimulation profoundly 

 modifies the activity of this process. Thus a moderately 



