120 



BOTANY 



found surrounding the individual vascular bundles, or particular rows of cells 

 provided with movable starch grains are present. The central cylinder of 

 Phanerogams is simple, and occupies a more or less central position. In a few 

 cases, as in the stems of Auricula and Gunnera, the central cylinder is broken 

 vip into several partial cylinders. 



In Roots, the division between primary cortex and central cylinder 

 is sharply marked by the endodermis, into which the innermost layer 

 of the primary cortex is usually transformed (Figs. 124, 129, 130 e). 

 The central cylinder becomes completely shut oft' from the primary 



cortex by the suberisation of the 

 lateral walls of the endodermal cells, 

 and by their close and uninterrupted 

 contact. While, by this means, the 

 passage of gases from the intercellular 

 spaces of the cortex into the central 

 cylinder, with the consequent obstruc- 

 tion of the water -channels, is pre- 

 vented, the passage of water from the 

 cortex to the central cylinder can, at the 

 same time, go on unhindered through 

 the unsuberised inner and outer walls 

 of the endodermal cells. In this 

 manner it is possible for the water, 

 absorbed from the soil by the root- 

 hairs or by the surface of the roots, to 

 Fir,. i3i.-i>art of a transverse section of \>Q transferred to the tissues of the 



a root of [rin Jlorentina. e, Endodermis, , i- j T ^v. u 



showing ceil walls thickened on one central cylinder. In the older parts 



side.; /, transfusion cell; p, pericycle ; of the TOOtS, which no longer absorb 



/; phloem ; s, vessel of xylem ; c, cortex. 

 (X 240.) 



water from the soil, the cells of the 

 endodermis become greatly thickened, 



but generally on one side only ; they may also be cutinised. 



Should thickening occur at an early stage, special endodermal 



cells, directly external to the xylem strands, remain unthickened 



and serve as TRANSFUSION CELLS (Fig. 131 /). 



While the root-hairs are as a rule developed from the cells of a definite region 

 of the epidermis, they may, in case the epidermis is thrown off at an early >t;igc, 

 arise from the outermost cortical layer, which then assumes the functions of an 

 epidermis. In any case the epidermis soon disappears, and the outermost cortical 

 layer becomes cuticularised and, as an EXODKKMIS, takes its place. Frequently 

 some of the cells of such an exodermis remain uucutinised and serve as transfusion 

 cells. They may be characterised by their smaller size, and be regularly 

 distributed between the cutinised cells. In aerial roots the epidermis usually 

 forms a many-layered root-sheath or velamen (cf. p. 109), within which comes 

 the exodermis. 



The primary cortex of the root is composed of colourless tissue, 

 which is usually parenchymatous. In the outer layers the cells are 



