SECT, i. ON VEGETATION. 169 



on account of the increase of chemical energy at that 

 time. To the same cause, the phosphorescence of certain 

 flowering plants and a few fungi, is supposed to be due. 



The action of heat is manifested through the whole 

 course of vegetable life, but its manifestations take 

 various forms suited to the period and circumstances 

 of growth. Upon it depends the formation of protein 

 and nitrogenous substances, which abound in the seeds, 

 buds, the points of the roots, and all those organs of 

 plants which are either in a state of activity, or are 

 destined to future development. The heat received, 

 acting throughout the entire organism of a plant, may 

 augment its structure to an indefinite extent, and thus 

 supply new instruments for the chemical agency of 

 light, and the production of new organic compounds. 

 The whole energy of vegetable life is manifested in this 

 production, and, in effecting it, each organ is not only 

 drawing materials, but power, from the universe around 

 it. The organizing power of plants bears a relation of 

 equivalence to the light and heat which act upon 

 them. The same annual plant from germination to the 

 maturation of its seed receives about the same amount 

 of light and heat, whatever be the latitude, its rate of 

 growth being in an inverse ratio to the amount it 

 receives in any given time. For one of the same 

 species, the more rapid the growth, the shorter the life. 



The living medium which possesses the marvellous 

 property of being roused into energy by the action of 

 light and heat, and which either forms the whole or 

 the greatest part of every plant, is in its simplest form 

 a minute globe consisting of two colourless transparent 

 concentric cells in the closest contact, yet differing 

 essentially in character and properties. The external 

 one, which is the strongest, is formed of one or more 

 concentric globular layers of cellulose, a substance 

 nearly allied to starch, being a chemical compound 



