XII. HOW PLANTS ARE MODIFIED BY THEIR SUR- 

 ROUNDINGS 



Problem XXI. How plants are modified by their surround- 

 ings. (Optional). (.Laboratory Manual, Prob. XXII.} 



(a) Hydrophytie society. 



(b) JCerophytic- society. 



(c) Mesopliytic society. 



(d) Plant societies. 



(e) Plant zonation. 



The Way in which Plants are Modified by their Surroundings. 

 As we have found in our experiments, young plants, and indeed 

 any living plants, are delicate organisms, which are affected pro- 

 foundly by the action of forces outside themselves. The presence 

 or absence of moisture starts or prevents growth in seeds or young 

 plants ; absence of light changes the form and color of green plants ; 

 a certain temperature, which varies for different plants, seems to in- 

 fluence plants in a healthy 

 growth. Pea seedlings 

 may grow for a time in 

 sawdust, but we know that 

 they will be much healthier 

 and will live longer if 

 allowed to germinate in 

 soil under natural condi- 

 tions. We are forced to 

 the conclusion that differ- 

 ences in the form and 

 habits of plants are caused 

 by the action of their sur- 

 roundings upon them. 

 The plants which have be- Pond lilieg> plants with floating leaves . Photo . 



Come in Various Ways fitted graph by W. C. Barbour. 



159 



