ADJUSTMENT TO WATER 



41 



tube, the root-hair. The cortical region consists of a varying 

 number of parenchyma layers. The cells, like those of the epi- 

 dermal layer, have thin cellulose walls, covered on the inner surface 

 by a thin layer of protoplasm. They are filled with water, but 

 usually lack other inclusions, although crystals are sometimes 

 present. Although the layers appear but slightly differentiated, 

 the outermost and innermost differ from the intermediate ones 

 in their final development. The outermost layer is the exoderm. 

 Its especial task is to replace the epidermis when the latter is 

 exfoliated, and accordingly to act as a root-hair producing surface. 

 The innermost layer is the endoderm. It is used for storage and 

 as a nutritive layer in the formation of lateral rootlets. When the 



pr. 



Fig. 12. Longisection of a root tip of the common dock, Rumex altissimus. 

 The primary regions are: d, dennatogen; pe, periblem; pi, pleronie; 

 ca, calyptra. The outer row of cells in the plerome is the pericyde, pr. 



other cortical layers have been exfoliated, in consequence of the 

 growth of the root in thickness, the cells of the endoderm divide 

 to form a several-layered corky covering for the root. 



The vascular region consists of a single fibro vascular bundle 

 surrounded by a layer called the pericyde. The latter, by the 

 periclinal division of several cells, produces the three primary laA'ers 

 of rootlets. By a similar division of the cells in front of the tra- 

 cheids of the bundle, it completes the ring of cam])ium which 

 makes possible the secondary growth of the root, i.e., its growth 

 in thickness. The bundle of the root is of the radial tyjoe. It 

 consists of woody tissue or xylem and sieve tissue or ])hloem. 

 These are separated by a meristematic tissue termed mesenchym. 



