PLANT'S GROWTH AND DECAY 105 



upon plant life, it seems fairly clear that the function of 

 the chlorophyl in the growing plant is practically three- 

 fold. It has been shown that it is those rays in the imme- 

 diate neighborhood of the red and orange in the spectrum 

 which most keenly excite the assimilation of carbon dioxide 

 and water vapor, and that the chloropyhl absorbs those rays 

 which hinder the formation of carbohydrates, transform- 

 ing rays of short wave lengths into those rays which most 

 favorably effect the production of the sugars and starch, 

 which are the food of the plant structure, and that it also 

 acts by the conversion of light into heat. 



The usual statement that the solid matter of the plant 

 consists of cellulose is, of course, only an approximation to 

 the truth, as cellulose is only one of several modifications 

 produced by the actions taking place in the growth of the 

 plant ; but as from a calorific point of view the other organic 

 bodies present have practically the same thermal value, it 

 is a convenient simplification to take wood as being com- 

 posed of cellulose, water, and the constituents of the sap, 

 mineral salts and extractive matters, which may be resinous 

 (as in coniferous woods), extractive (as in beech or birch) 

 .or tannin (as in oak). 



The chemical actions which have resulted in the forma- 

 tion of the cellulose have required an expenditure of 

 energy which, in the primary decomposition of the carbon 

 dioxide and w-ater vapor, can be expressed in terms of the 

 heat necessary to raise a unit weight of water one degree. 



As in the growth of the plant this energy has been de- 

 rived from the sun, and has been partially rendered latent 

 in the cellulose, when we burn that compound in the form 

 of wood so as again to convert the carbon and hydrogen to 

 carbon dioxide and water vapor, we once more set free the 

 stored energy in the form of heat and can render it avail- 

 able for heating purposes or for doing work. 



The variations in the physical properties of wood are 

 dependent upon the constituents of the sap and the density 

 with which the solid matter is packed away in the struc- 



