GROWTH 



agents known. The majority of animals and plants, 

 however, require an abundance of light for normal 

 growth and development. Seedlings grown in the 

 dark and those grown in diffuse light present a 

 striking contrast. The former are " spindling," 

 weak and pale, and always much larger than those 

 grown in the light. Even diffuse light thus seems 

 to have a retarding effect on growth as such, although 

 necessary for the normal development of the plant. 

 The difference in size between plants grown in the 

 dark and those grown in the light is perhaps due to 

 the loss of water that takes place in the light, with 

 a consequent concentration of protoplasm. This 

 idea is supported by the fact that aquatic plants and 

 animals grow faster in the light than in the dark, 

 owing doubtless to the fact that such a loss of water 

 does not occur. 



The white light of the sun may be broken up into 

 the components of the spectrum, and it has been dis- 

 covered that the various-colored rays of the spectrum 

 have very different effects upon growth. The actinic 

 (ultra-violet) rays are the most active, and it is 

 probably their presence that makes direct sunlight 

 so destructive to living matter. The action of the 

 other rays varies, but in some plants there is a pro- 

 gressive inhibition of growth from the red end of the 

 spectrum to the other. 



Temperature. Various different organisms are to 

 be found thriving in all extremes of heat and cold 

 between C. and a very little below the boiling 



