174 TEXTBOOK OF BOTANY 



slender roots that often reach to a great depth. Thus it is 

 reported that old, well-grown trees may send their roots 

 to a depth of sixty feet, provided the soil is deep enough 

 and otherwise favorable for the growth of the roots. 

 Annuals and small perennials have much shorter roots ; still, 

 the root systems of barley and mustard reach a depth of 

 about three feet, those of clover and wheat a depth of six 

 feet, and those of alfalfa, it is said, may extend nine feet 

 below the surface of the ground. It is plain that, other 

 things being equal, the plant whose roots grow to the greatest 

 depth has the best chance of living through a dry season. 



196. Conduction. The water and other substances 

 taken in by the root hairs must be carried from the extremities 

 of the root system through the whole length of the branch 

 roots and of the main root to the point where the latter 

 joins the stem. All parts of the root system must also be 

 supplied with food that has been manufactured in the parts 

 of the plant above ground. The conduction of the sap and 

 of manufactured foods goes on through the vascular bundles. 

 Thus absorption and conduction in roots are closely related ; 

 the more extensive the root system becomes, the farther is 

 the younger, absorbing portion of each root from the stem, 

 and the farther therefore must sap and manufactured foods 

 be carried. 



197. Attachment and Support. There are some plants, 

 such as the duckweeds which float upon the surfaces of ponds 

 and lakes, whose roots are merely absorbing and conducting 

 organs. But as a rule plants depend upon their roots also 

 to attach them to the soil or to the other substances on which 

 they live. If the stem is above ground and erect, the roots 

 are also a means of supporting the plant in its erect position. 

 The larger the plant, naturally, the more important is the 

 work of attachment and support. It is least important in 

 plants whose stems, like that of the may-apple, are under- 

 ground. The extensive, much-branched root system, which is 



