XII. HOW PLANTS ARE MODIFIED BY THEIR SUR- 

 ROUNDINGS 



XXI. Unit- />/frnfx f/rc inotlijicil hi/ ////// xnrround- 

 ings. (Optional). (Laboratory Manual, Proh. ,\'.\ 7. 



(a) Hydrophytic .w/Y///. 



(b) XeropUytic society. 



(c) Mesophyti>c society. 

 07) 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- 

 come in various ways fitted 



Pond lilies, plants with flouting leaves. 

 graph by W. C. Barbour. 



159 



