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TEXTBOOK OF BOTANY 



Fig. 121. Photomicrographs of cross sections of roots of a tuhp tree {Lirioden- 

 dron) in which the arrangement of tissues is evident: A, a young root; B, a 2- 

 year-old root. 



The terminal portion of the root, then, is the zone of origin of cer- 

 tain root tissues, the zone of growth in length and of growth curvature, 

 and the principal zone through which water and salts enter many kinds 

 of plants. 



Tissues of roots. When longitudinal sections of the growing tips of 

 stems and roots are compared, certain similarities and differences are 

 easily recognized. The three zones characterized in general by cell 

 division, enlargement, and differentiation (maturation); and the three 

 longitudinal cylinders represented by the epidermis, cortex, and vascular 

 cylinder are common to both. In some roots these three cylinders are 

 distinguishable all the way to the so-called growing point in the center 

 of the zone of cell division, which lies just above the root cap ( Fig. 123 ) . 

 In other roots the differentiation of cells in these three regions is not 

 obvious so near the growing point. 



The four regions (root cap, epideraiis, cortex, and primary vascular 

 axis) of the growing tip of the root all originate from cells formed in 

 this growing point. The root cap is a cup-shaped mass of cells renewed 

 by cell division and enlargement at the lower side of the growing point 

 and sloughed off on the outer surface by disintegration of the middle 

 lamellae of the cell walls, accompanied by abrasion when the roots are 

 growing in the soil. 



In the zone characterized by cell enlargement there are also some cell 



