ECOLOGY 147 



are usually to be classed with mesophytes, because they 

 are subject to much fluctuation in the water-supply. The 

 soil may be saturated at one time and moderately dry 

 at another time. Swamp-grasses often show xerophytic 

 characters, especially roll-leaves. During the early part 

 of the growing season, particularly in the North, the air 

 at least during the day is warm while the roots are im- 

 mersed in the cold substratum. There is thus danger of 

 the loss of water by evaporation from the leaves faster 

 than the cold sluggish root-system can supply it; hence 

 the presence of roll-leaves. 



GEOGRAPHICAL DISTRIBUTION 



187. Geographical distribution of plants is their 

 range or dissemination over the surface of the earth. 

 The present distribution is the result of causes which 

 have acted through an indefinitely long period of time and 

 often over areas of continental extent. Every species of 

 plant occupies its present area by virtue of its ability to 

 adapt itself to its environment. If the environmental 

 conditions change, the plants concerned must become 

 adapted to the new conditions, or they are forced to mi- 

 grate, or, failing in this, they become extinct. It is not 

 the purpose here to discuss the causes that have brought 

 about these changes, but merely to outline the present 

 distribution of the grass family. For a further discussion 

 the student is referred to the works dealing with the 

 evolution of plants, especially those of Darwin, Wallace, 

 Hooker and, Gray. 



Darwin: "Origin of Species." Wallace: "Darwinism," "Island 

 Life," "The World of Life," and other works. Hooker: "Distri- 

 bution of Arctic Plants." Gray: "Collected Essays." 



