130 



BOTANY 



PAKT I 



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. lu a few cases, as in the stems of Auricula and Gtmnera, the central 

 cylinder is broken up 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. 134, 139, 140 e). 



Fio. 140.— Transverse section of an adventitious root of Allium Cepa. c, Primary cortex ; e, endo- 

 dermis ; p, pericycle ; a, annular tracheides ; $p, spiral tracheides ; sc, and sc*, scalariforra 

 vessels; v, phloem, (x 240.) 



A close and uninterrupted contact of the endodermal cells is brought 

 about by the chemical change which the middle strip of their lateral 

 walls undergo. This results in a shutting off of the central cylinder 

 from the air-containing intercellular spaces of the cortex. In the 

 older parts of the roots, the cells of the endodermis become greatly 

 tliickened, but generally on one side only. Should thickening occur 

 at an early stage, special endodermal cells, directly external to the 

 xylem strands, remain unthickened and serve as TRANSFUSION cells 

 (Fig. 141/). 



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

 of the epidermis, they may, in case the epidermis is thrown off at an early stage, 

 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 more or less cuticularised and, as an exouermis, takes its place 



