GROWTH AND DEVELOPMENT 19 



root systems which store up water in great quantities and the plants 

 depend upon these supplies during rainless seasons. 



Roots have obvious advantages as storage organs because of their 

 position. They are better protected from animals than are aerial 

 organs and are somewhat protected also from extremes of tempera- 

 ture or, perhaps of more importance, from sudden changes of tem- 

 perature. Furthermore, and of still greater importance, they are 

 very well protected from desiccation. 



8. Growth and Development.— Certain factors in the environ- 

 ment of roots are more constant than corresponding factors in the 

 environment of aerial organs. This is especially true of temperature. 

 Some other factors, however, are as variable below the surface of 

 the soil as above it. In the case of plants that live only during one 

 season the roots ordinarily grow continuously from the time the 

 seed germinates until the plant dies, although the rate of gro^^i:h 

 varies greatly from time to tune as the factors of the environment, 

 such as temperature and water supply, become more or less favor- 

 able. The roots of plants that live more than one year ordinarily 

 cease growth at certain times during each year, at least in regions 

 having warm and cold or wet and dry seasons. These periods of 

 gro^^i;h cessation are spoken of as "rest periods" though, of course, 

 the roots are never actually in need of rest from growth. The cause 

 of these rest periods is lack of sufficient water for growth. In the 

 case of forest trees in the temperate zones, for instance, there is 

 always a rest period in winter when the soil becomes physiologically 

 dry through low temperature, and in a dry year there may also be 

 a rest period in summer when the soil becomes physically dry. In 

 a w^et year, however, the roots of trees grow throughout the summer 

 and this is probably the rule every year in the case of those roots 

 that are so deep in the soil that the water supply never becomes 

 inadequate. 



The characteristics of a root system as to shape and extent depend 

 in part upon inheritance and in part upon environment. "Water is 

 the most important environmental factor involved. In the tall 

 grass prairies of eastern Iowa, Nebraska and Kansas, where the 

 rainfall is sufficient to wet the soil to great depths, the most charac- 

 teristic grasses, such as bluestem (Andropogon), wheat grass {Ayro- 

 pyron), and marsh grass (Spartina), all have root systems that 

 extend from 5 to 8 feet into the soil. Some of the other important 

 plants of the same community, such as sunflowers, asters, golden- 

 rods, mints, and roses, send their roots even deeper and often absorb 



