ROOTS AND THEIR FUNCTIONS 



143 



known as a fibrous root system, as in many grasses. Other types of root 

 systems also exist. In many plants, the primary root system is supple- 

 mented or superseded by the development of numerous adventitious or 

 accessory roots, which may develop from any part of a plant. They are 

 especially numerous on underground stems, but they may also arise from 

 the leaf nodes of aerial stems or even from leaves themselves. 



The root tip. The apical portion of the root as far back as the base 

 of the zone of root hairs (generally 13^ to 2 inches in length) constitutes 

 the root tip. This is the part of the 

 root that grows in length, produces 

 new tissues, and carries on absorp- 

 tion of soil moisture and dissolved 

 substances. Beginning at the apex, 

 four regions may be recognized in 

 the root tip: the root cap, the grow- 

 ing point, the region of elongation, 

 and the region of maturation, or zone 

 of root hairs. 



The growing point is a part of the 

 meristem. It is composed of embry- 

 onic tissue — undifferentiated cells 

 that grow and divide very rapidly. 

 The divisions occur mostly at right 

 angles to the axis of the root, so that 

 cells are cut off from the meristema- 

 tic zone alternately on the side to- 

 ward the tip of the root and on the 

 side toward the base. Those pro- 

 duced on the side toward the tip are 

 added to the root cap and make 

 good the constant loss of its outer 

 layers caused by abrasion. Those 

 cut off on the side toward the base of the root are destined to form the 

 various tissues of the mature root. Between these two regions of 

 differentiating daughter cells, the parent meristematic zone remains as 

 the persistent growing point, continuously shifted forward by its own 

 active cell division. 



As the growing point advances away from the cells cut off behind it, 

 those cells first elongate and later begin to broaden out and to differentiate 

 into the various sorts of tissue cells characteristic of the fully formed root. 

 This sequence of changes is responsible for the zonal arrangement of the 

 * cells of the root tip. The surface layer of cells forms the epidermis, one 

 cell in thickness. These cells are roughly cuboidal in shape and are thin- 



Fig. 10.1. Comparison of fibrous and tap- 

 root systems. In the grass (left) the roots 

 are slender, much branched, and mostly 

 shallow. In the beet (right) there is a single 

 large, deep taproot from which lateral roots 

 arise. The enlarged base of the root serves 

 for food storage. 



