MORPHOLOGY 



113 



(Fig. 129, /). While the root-hairs are as a rule developed from the cells 

 of the epidermis, they may, in case the epidermis is thrown off with the 

 root-cap, arise from the outermost cortical layer, which then assumes the 

 functions of an epidermis. In any case the epidermis ultimately dis- 

 appears, and the outermost cortical layer becomes cuticularised and, as an 

 exodermis, takes its place. In aerial 

 roots the epidermis may become con- 

 verted into a many-layered tracheidal 



ROOT-SHEATH (p. 100). 



The PRIMARY CORTEX OF ROOTS is 



composed of colourless tissue which, 

 with few exceptions, consists wholly 

 of parenchyma. Although the cells of 

 the outer layers of the cortex are un- 

 interruptedly in contact with one 

 another, the inner layers are often 

 provided with intercellular air cavities 

 or passages. 



The outer layer of cells (Figs. 

 120, 126, 129, p) of the central 

 cylinder OF ROOTS often forms the 

 pericycle (pericambium 

 consists of a single 



this Usually Fio. 129.— Part of a transverse section of 

 lenrpr nf nplls a. root of Iris florentina. e, Endoclermis, 



layer 01 ceils, s h ow ing ce n , va n s thickened on one 



but may be many-layered or entirely side ; /, transfusion ceil ; p, pericycle ; 



absent. The xylem and phloem », phloem ; s, vessel of xylem ; c, cortex. 



portions of the xylem bundles of 



roots form separate strands (p. 103), radially disposed and alternat- 

 ing with each other (Figs. 120, 126). It has already been shown 

 that the narrowest elements of the vascular strand are outermost. 

 Eoots are described as diarch, triarch, polyarch, according to the number 

 of the radiating vascular strands. For example, the roots of J corns 

 Calamus (Fig. 120) are octarch, those of Allium Cepa (Fig. 126) hexarch. 

 The vascular strands may either meet in the centre (Fig. 126) or they 

 may surround a central pith (Fig. 120). Like the corresponding 

 tissue in the stem, the fundamental tissue between the xylem and 

 phloem strands may be termed primary medullary ray tissue. 



There is never more than one central cylinder in a root ; in the tubers of 

 Orchids the apparently large number of such cylinders may, on phylogenetic 

 grounds, be considered as having resulted from a fusion of an equal number of 

 roots. 



Leaves consist chiefly of fundamental tissue. This tissue is a con- 

 tinuation of the fundamental tissue of the primary cortex, and is termed 

 the mesophyll. It is traversed by vascular bundles, and covered ex- 

 ternally with an epidermis. As the vascular bundles on entering a 

 leaf are accompanied by fundamental tissue from the central cylinder 



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