THE ROOT. 67 



the outside parts become fissured lengthwise. Thus, in an 

 old and large root of the woody type, all the parts outside the 

 central wood constitute a bark, which becomes furrowed 

 lengthwise, like the bark of the stems of many trees. Such 

 secondary thickening finally produces in the roots a structure 

 which is almost identical with that of stems which have under- 

 gone secondary thickening. (Compare Tf iii.) 



80. (b) Fleshy roots. — But if thin-walled cells are the 

 chief products, the root often becomes very thick and fleshy, 

 as in the carrot, turnip, radish, sweet potato, beet, dahlia, 

 artichoke, etc. Such roots serve the plant as storehouses of 

 reserve food, and are consequently useful to animals as food. 

 This thickening for storage purposes may affect either the 

 primary or secondary roots, or both. 



81. (c) Float roots. — Plants which grow in water or in 

 very wet swamps sometimes modify their roots to serve as 

 floats. In these cases, the voluminous cortex consists of large 

 cells, with huge intercellular spaces which are filled with air. 

 The root thus serves to buoy up the parts of the plant to 

 which it is attached, and assist in its respiration. (See ^ 

 202.) 



82. (d) Tendrils, thorns, etc. — In a very few plants, 

 aerial roots are modified into tendrils, being slender, sensitive 

 to contact, clasping the objects which they touch, if of suit- 

 able size, and thus assisting the plant to climb ; in some in- 

 stances they are altered into thorns, being short, rigid, and 

 sharp-pointed ; in others, being exposed to the light, they 

 develop chloroplasts, which enables them to act as organs for 

 the manufacture of food. 



83. Branching. — Both primary and secondary roots may 

 branch. The mode of branching is commonly monopodial, 

 i.e., the central axis grows most vigorously, and bears lateral 

 branches upon its sides. The normal branches arise from 

 lateral growing points, which originate in regular succession 



