CHAPTER X 



ROOTS AND THEIR FUNCTIONS 



We ordinarily think of a root as being any part of a plant that is buried 

 in the soil. Actually many subterranean plant structures are not really 

 roots, and not all roots are buried. A true root has a number of identifying 

 characteristics. It does not possess nodes bearing leaves or leaf buds; its 

 tip is covered by a special root cap; the internal arrangement of its tissues 

 differs from that in a stem ; and it generally possesses root hairs in a zone 

 just back of its tip. Furthermore, the branches of roots do not develop 

 from buds, as do those of stems. Instead they originate from a layer of 

 cells within the root, and push their way out through the overlying 

 tissues to the surface. 



A typical root is almost cylindrical, tapering gently from the base to 

 the free end. The root cap at its tip protects the delicate tissues of the 

 growing point from abrasion as the root pushes through the soil. The 

 zone of root hairs a short distance behind the root tip is the region where 

 absorption of soil solutions occurs. Back of this zone, the root does not 

 increase in length but only in thickness; the primary functions of this 

 older part of the root are the conduction of absorbed materials, anchorage 

 of the plant, and the production of branch roots. 



The radicle of the seed develops into the -primary root, which tends to 

 grow straight downward. In many plants, this primary root gives rise 

 to the entire root system, by sending off secondary lateral roots. These 

 develop in regular succession from above downward; since they originate 

 in a definite position within the primary root (generally opposite the 

 xylem masses, described below), they tend to be arranged in longitudinal 

 rows. True forking of roots is unknown in the higher plants, though 

 subsequent enlargement of some of the branch roots may simulate fork- 

 ing. In plants where the primary root continues to elongate, producing 

 lateral branch roots in regular succession (an inverted counterpart of the 

 conical trunk-and-branch pattern often seen in the aerial parts of plants), 

 the arrangement is called a taproot system, exemplified in pine, dandelion, 

 clover, and carrot. In plants in which the primary root soon ceases to 

 grow, the major part of the root system is formed from many thin, nearly 

 equal branch roots developed from the short axis, producing what is 



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