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.” 
