FERNS, FOSSILS AND FUEL 
meadow, swamp, or desert. In the same way, plants, like 
all other living organisms, must have certain things that 
make their life possible. The most important of these are 
water, sunlight, temperature of the air, nutritious soils, 
and frequently an object to which to cling or which will 
furnish protection against excessive sunlight. These are 
some of the ecological factors of plant life, and the science 
which deals with them we call plant ecology. 
During the unfolding of geologic time, the most im¬ 
portant changes for plant life were in climatic conditions, 
involving primarily temperature and humidity. Every 
plant species requires a minimum temperature and a mini¬ 
mum amount of moisture for its existence, and cannot 
live beyond a maximum temperature and maximum mois¬ 
ture. These margins of safety vary with nearly every 
species. We find, therefore, different groups or associa¬ 
tions of plants in a tropical jungle, in an Alpine valley, 
in a desert, or in a swamp. The floras of the earth dur¬ 
ing geologic times reflect the changes of climate during 
those periods. 
The climate of the earth in past ages was normally warm 
and rather uniform, and periods of glaciation or extreme 
cold were very much the exception. The amount of mois¬ 
ture must have varied greatly, but the warm climate was 
usually accompanied by a sufficient rainfall, and a great 
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