No. 17] FLOWERING PLANTS AND FERNS 17 
While vegetation is not of equal vigor and variety in the 
frozen wastes surrounding the poles, it is not entirely absent. 
In spite of forbidding climatic conditions, lichens encrust the 
rock surfaces, and the snows are discolored by minute forms 
called algae. 
Temperature is but one of the important factors influenc- 
ing the distribution of plants. The Sahara Desert and the 
luxuriant jungles of Central America are both in the tropics, 
with approximately the same temperatures. The difference 
is due to absence of moisture in the Sahara and excess of rain- 
fall in Central America. Contrast the abundance and variety 
of vegetation in Mississippi with the arid conditions in Death 
Valley, California, both of which are in about the same latitude. 
The difference here is due to difference of rainfall in the two 
regions. Hence, we are impressed with the very great import- 
ance of moisture as a factor in plant distribution. 
Then again, in Mississippi we have the extensive pine 
forests of the southern counties, the enormously valuable hard- 
wood forests of the Yazoo-Mississippi Delta, and the broad, 
gently rolling treeless prairies of the northeastern counties. 
These are natural and marked distributional features of vegeta- 
tion in our own state. This distribution is not due to differ- 
ences of temperature or of rain-fall, but to differences of soil 
conditions. Hence, we have forced upon our notice the im- 
portance of soil conditions as a factor in plant distribution. 
Ecology.—This brings us to a consideration of Ecology. 
Plants are not distributed by haphazard, but, as we have seen, 
certain casual factors operating upon them determine the 
limits and bounds of their distribution. These are called 
Ecological Factors, and Ecology may be called plant Sociology, 
o1 the association together of plants of one or more species— 
often of many species—under conditions favoring their devel- 
opment forced upon them by the operation of all the ecological 
factors to which they are subjected. In other words, the sum 
of. the ecological factors determines both the habitat and the 
associates of a plant. Groups of plants thus brought together 
