CONDITIONS AFFECTING GROWTH 299 



conditions for germination, and note the comparative periods of 

 rest shown by the specimens kept warm, and those given an ex- 

 posure to low temperatures. The specimens placed in the warm 

 room should be packed in damp moss to prevent desiccation. 



378. Conditions Affecting Growth. Temperature, electricity, 

 food supply, free oxygen, moisture and turgidity of the plant, 

 barometric pressure, mechanical shock, traction and compression 

 affect the rate of growth. So far as the trophic factors are con- 

 cerned it is to be said that there is a certain optimum intensity at 

 which growth proceeds most rapidly, and any deviation from this 

 will check increase in the body of the plant. Light has hitherto 

 been regarded as exercising a retarding or paratonic effect upon 

 growth, but as the result of recent investigations it is known to 

 influence growth only indirectly by its effect upon the food-form- 

 ing processes, and upon other kinds of metabolism. A number 

 of instances are proven in which light accelerates growth, but 

 the probability is not excluded that growth of some species of 

 special habit may be retarded by the action of light (Chapter 

 VIII.). 



The temperature at which growth proceeds most rapidly varies 

 with the species (Chapter VI.). Infrequent electric currents in- 

 crease the total amount of growth (Chapter VII.). A proper food 

 supply, either as a reserve or available in the substratum, must be 

 available to form constructive material for growth. Hydrostatic 

 pressure or turgidity is necessary for the growth of almost all cells, 

 although a few instances are known in which increase in volume 

 may be carried through a wide range without this force. Mechan- 

 ical stimuli of all kinds decrease the rate of growth when first ap- 

 plied, but later the rate of growth may exceed that of the untreated 

 specimen or in some instances fall below it (Chapter II.). 



Growth implies more or less destructive metabolism and the 

 liberation of energy, which is chiefly accomplished by means of 

 combination with free oxygen, and this condition is necessary for 

 all organisms except the anaerobes which use other forms of 

 respiration. Variations caused by alterations in atmospheric 



