WARMTH 501 



to a particular environineiit. Wc are constantl}^ 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 prol)lems 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 adj usted 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 crop 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- 



