DIV. I 



MORPHOLOGY 



135 



FIG. 15y. Transverse section of an adventitious root of 

 Allium Cepa. ep, Remains of the epidermis ; ex, exo- 

 dermis ; o, primary cortex ; e, endodermis ; cc, central 

 cylinder, (x 45. After M. KOERNICKE.) 



the root-hairs. The outermost layer of the cortex then forms a cutis- 

 tissue called the EXODERMIS ( 70 ) on the surface, the cell walls 

 becoming more or less suber- 

 ised (Fig. 159 ex). 



Some of the cells of the exoder- 

 mis often remain unsuberised and 

 serve as transfusion cells. They 

 are regularly placed among the 

 corky cells and smaller than the 

 latter. 



The remaining tissues of 



the root can be distinguished 



into cortex and central 



cylinder. 



The primary cortex of 



the root is composed of 



colourless tissue, which is 



usually parenchymatous. In 



the outer layers the cells 



are in close contact with one 



another, but intercellular spaces are present more internally. These 



intercellular spaces often widen into air-cavities or passages. In 



many roots a hypoderma giving 

 mechanical support to the epi- 

 dermis or exodermis is present. 

 The innermost layer of the 

 cortex is usually developed as 

 an ENDODEKMIS ( 71 ) (Figs. 159, 

 160 e, 161 S, 163 *), which 

 sharply marks the limit between 

 cortex and central cylinder. 

 The endodermis consists of 

 somewhat elongated, rectan- 

 gular, prismatic cells which in 

 transverse sections show the 

 dark Caspary dots on their 

 radial walls. The nature of 

 these strips of the wall (cf. p. 



Fi<;. MO. Transverse section of central portion of 58) shuts off to SOme extent 

 the root of Acorus Calamus, m, Medulla ; *, xyleni; tne cen tral Cylinder from the 

 r, phloem ; p, pericycle ; e, endodermis ; c, cortex. . J . . , 



(XPO. After STRASBURGER.) primary cortex ; the tangential 



walls of the young endodermal 

 cells, however, allow of passage of water between the two regions. 



In the older parts of the roots the cells of the endodermis become corky, and in 

 many Monocotyledons are greatly thickened, but generally on one side only. 



