28 GROWTH AND WORK OF PLANTS 



and have little difficulty in obtaining the necessary water in dry 

 soils or in dry seasons. Cone-bearing trees, the pines, spruces, 

 etc., have a shallow root system, and are especially suited to 

 growing in regions where plant food is chiefly confined near the 

 surface of the ground. Plants growing in the desert, except the 

 annuals, which grow only during the rainy season, usually have 

 a root system which extends to a considerable depth in the 

 soil. 



47. The mesquite tree of the Southwestern States and Mexico 

 is a remarkable example of the relation of the roots to the soil 

 under different conditions. Where the soil is not very dry it 

 forms a large tree and the roots do not extend very deeply into 

 the soil. In the very dry regions, however, the tree attains a 

 height of only two or three feet and it extends its roots very 

 deeply into the soil, sixty feet or more, to obtain water. 



KINDS OF ROOTS. 



Besides the variation in the root system of plants, there are 

 several kinds of roots which do special work for the plant. 



48. Aerial roots. These are most common in the case of 

 many tropical plants which grow on trees. Such plants are 

 epiphytes. The roots here serve as grapplers to attach the plants 



Fig. 33- 

 Aerial roots of wandering Jew (Tradescantia). 



to the limbs and trunks of trees. Good examples of these can be 

 seen in the case of many tropical orchids which are grown in so 

 many greenhouses. Some of the roots dangle in the air and are 

 provided with a special sheath of spongy tissue (the vela men) 



