ROOTS 



255 



into a cluster of branches (Fig. 219, B). In the former case, 

 the plant is said to have a tap-root; and in the latter case, 

 when, as in grasses, the clustered branches are slender, the 

 plant is said to have 

 fibrous roots. In both 

 cases the root may be- 

 come enlarged in con- 

 nection with food stor- 

 age, and many of our 

 common vegetables are 

 such thickened roots. 

 For example, radish 

 (Fig. 220, A), turnip, 

 and parsnip are thick- 

 ened tap-roots ; while 

 sweet potatoes are the 

 thickened branches of a 

 cluster of roots, as in 

 the dahlia (Fig. 220, 

 B). It is evident that 

 thickened roots, just as 

 thickened underground 

 stems ( 136, p. 250), 

 enable the plant to de- 

 velop its aerial parts 

 much more quickly than 

 by the method of seed- 

 germination. 



139. Root-cap. The 

 growing tip of each root 

 and rootlet is protected 

 by a cap of cells called the root-cap (Fig. 221, c). This 

 root-cap consists of several layers of cells, the outer ones 

 gradually dying or being worn away as the tip of the root 

 pushes through the soil, and being replaced by new layers 



FIG. 221. Longitudinal section through root- 

 tip of spiderwort, showing central vascular 

 cylinder (pi), cortex (p), epidermis (e), and 

 root-cap (c). 



