ROOTS: KINDS OF ROOTS 



33 



near the surface are the first to suffer. Hickory and walnut trees 

 have a tap root which extends to great depths in the soil, 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. 



54. The mesquite tree of the Southwestern States and Mexico 

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

 der 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. 



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

 many tropical plants which grow on trees. Such plants are 



Fig. 36. 

 Aerial roots of wandering Jew (Tradescantia). 



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

 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 



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