THE A'OOT. 



77 



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

 anything is left of it, constitute a bark, which becomes fur- 

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

 Such secondary thickening finally produces in the roots a 



FIG. 91. A, diagram of primary structure. /?, C, diagrams showing the results of 

 secondary thickening from the stelar cambium in the two extreme forms c, cortex; 

 ed, endodermis ; /, pericycle ; /, primary phloem ; /', secondary phloem; />, primary 

 xylem ; />', secondary xylem ; .i,-, stelar cambium; r, secondary pith-rays; /;/, pith. 

 After Van Tieghem. 



structure which is almost identical with that of stems which 

 have undergone secondary thickening. (Compare ^j 133.) 



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

 predominant products of the stelar cambium, 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. 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 



