PLANTS MODIFIED BY THEIR SURROUNDINGS 169 



like the aspen and wild cherry, which have the birds to help them 

 out, invade the territory. Eventually we may have the area re- 

 tenanted by its former inhabitants, especially if the destruction of 

 the original forest was not complete. 



In like manner, on the upper mountain meadow or by the sand 

 dunes of the seashore, wherever plants place their outposts, the 



A plant outpost. The struuirle here is keen. The advancing sand has 

 killed the trees in the foreground. 



advance is made from some thickly inhabited area, and this advance 

 is always aided or hindered by agencies outside of the plant the 

 wind, the soil, water, or by animals. Thus the seeds obtain a foot- 

 hold in new territory, and thus new lands are captured, held, and 

 lost again by the plant communities. 



REFERENCE BOOKS 

 ELEMENTARY 



Sharpe, A Laboratory Manual for the Solution of Problems in Biology. American 



Book Company. 



Andrews, Botany All the Year Round. American fiook Company. 

 Bergen and Davis, Principles of Botany. Ginn and Company. 

 Coulter, Plant Relations. D. Appleton and Company. 

 Leavitt, Outlines of Botany. American Book Company. 

 Stevens, Introduction to Botany. D. C. Heath and Company. 



ADVANCED 



Clements, Plant Physiology and Ecology. Henry Holt and Company. 

 ( '(.alter, Barnes, and Cowles, A Textbook of Botany, Vol. II. American Book Com- 

 pany. 



Kerner, Natural History of Plants. 4 vols. Henry Holt and Company. 

 Schimper, Plant Geography. Clarendon Press, 



