WARMTH 501 
to a particular environment. We are constantly striving to 
find the Apples, Pears, and other fruits best adjusted to the 
environmental factors of different regions. One of the objects 
in the breeding of Citrous fruits has been to procure varieties 
less sensitive to cold, so that Citrous fruits can be grown farther 
north and consequently over a larger area. Much time and 
energy has been spent in obtaining strains of Cotton resistant 
to the insect pests and other unfavorable environmental factors 
of the Southern states. In the Northern states, where the 
growing season is short, one of the problems in connection with 
the raising of Corn is to secure varieties that can mature before 
frost. The securing of drought resistant plants for dry regions, 
of plants resistant to the diseases prevalent in the different 
agricultural regions, of pasture Grasses best adapted to a given 
region, of trees adapted to grow in a given region for shade or on 
a given area that is to be reforested are some of the many other 
agricultural problems that have to do with adjustment of plants 
to their environment and hence are ecological. 
Ecological Factors 
The various environmental features to which plants and ani- 
mals must adjust themselves are called ecological factors. The 
chief ecological factors are water, heat, light, soil, wind, and 
associated plants or animals. 
Water. — This is one of the most important ecological factors. 
The amount of water to which various plants are adjusted varies 
from complete submergence to perpetual drought. Most Algae 
live completely submerged in water, while Cacti are adjusted 
to the drought of deserts. Most crop plants require a medium 
amount of water in the soil, and an excess or lack of water re- 
tards their growth. But among ercp plants there is also much 
variation in the amount of water necessary for living. For 
example, the Sorghums are more resistant to drought than 
Corn, while some varieties of Rice require flooding. 
Warmth. — All kinds of plants are adjusted to certain ranges 
of temperature. For example, Wheat and Oats require less 
warmth than Corn, and hence can be grown farther north. 
There are great zones of plants corresponding to the great 
zones of temperature. Thus the arctic, temperate, and tropi- 
