THE PLANT AND ITS ENVIRONMENT 181 



REFERENCE PROBLEMS 



80. The mean annual temperatures ("average" temperature) of England 

 and of New England are not very different. Many delicate plants will grow 

 in the former region but not in the latter. How do you account for this? 



81. Compare the United States and the Britisii Isles as to rainfall and 

 temperature during the growing season. How do these figures explain the 

 fact that grass grows so much better there than here, and corn so much 

 better here than there? 



82. What is meant, in ecology, by a succession of plant associations and 

 what causes such a succession? 



83. Although a much wider range of plants can Ijc grown in northern 

 Europe than in the northern United States, the latter region has far more 

 native species than the former. Explain. 



84. As a general rule, which can be used in agriculture over a wider 

 area a breed of livestock or a variety of cultivated plants? .Wliy? 



85. Give the derivation of the following terms and explain in what way 

 each is appropriate: 



Phototropism Etiolation Parasite 



Geotropism Xerophyte Sapropiiyte 



Hydrotropism Hydrophyte Epiphyte 



Chemotropism Mesophyte Symbiosis 



