THE ROOT. 



77 



and the phloem region, nith the cortex between them, if 

 anything is left of it, constitute a I/ark, which becomes fur- 

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

 Such secondary thickening finally produces in the roots a 



Fig. qi.—A. diagram of priman' structure. P. C. diagrams shon-ing the results of 

 secondare' thickeniug from the stclar cambium in the two extreme forms c. cortex ; 

 e^i. dudodermis : /-, pericycle ; /-h' . primary phloem ; /A", secondary phloem ; .r'* 

 primary xylem ; .r", secondary xylem ; tY', stelar cambium : r', secandar>' pitli-rays ■ 

 I", pith.— After Wan lieghem. ' 



Structure which is almost identical with that of stems which 

 ha\'e undergone secondary thickening. . (Compare % 133-) 



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

 predominant products of the stclar cambium, the root often 

 becomes very thick and fleshv, as in the carrot, turnip, 

 radish, sweet potato, beet, dahlia, artichoke, etc. Such 

 roots serve the plant as storehouses of reserve tbod, and are 

 consequently useful to animals as food. The thin-walled 

 cells which are produced in such volume may belong to the 

 phloem region, as in the carrot and parsnip, or to the xylem, 

 as in the radish and turnip. This thickening for storage 

 purposes may affect either the primary or secondary roots, 

 or both. Other plants may develop the cortex (orchids) or 

 the pith (daffodils) to an extraordinary degree, forming 

 fleshy roots which also function as storehouses. 



89. (c) Float roots. — In plants which grow in water or 

 in very wet swamps, roots are sometimes modified to serve as 

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



