XII. HOW PLANTS ARE MODIFIED BY THEIR SUR- 

 ROUNDINGS 



Problem XXI. Horu plants (tre iiindifietl hy thrir surround- 

 ings. (.Optional). {Laboratory Manual, I'roh. XXI.) 



(a) Hydrophytic society. 



(b) .Verophytio society. 



(c) Jlcsophytic 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- „ , ,.,. , . -.i, n *■ i wi * 



^ Pond lilies, plants with floatinK Iravcs. Photo- 



COme in various ways fitted graph by W. C. Barbour. 



159 



