STRUCTURE. 31 



In all cases, however, the epidermis persists on the narrower portion of 

 the root within about 2.5 cm. of the rhizome. Its loss is the direct result 

 of the great expansion of the root, caused by the enlarging and maturing 

 of the air-canals. 



Meanwhile the subjacent outermost layer of cortex has developed in 

 an especial manner (see Fig. 7 //). At a very early stage this layer is 

 composed of columnar cells closely resembling the epidermis at the same 

 period. But as its cells become broader and more nearly cubical, the 

 walls become collenchymatously thickened, as also do those of the next 

 one or two layers below (TV. lotus, tuberosa, marliacea-chromatella}. The 

 outer walls of the hypodermal cells unite to form a fairly even surface, 

 but the inner walls meet at various angles, owing to the varying depths 

 of the cells. Thus an even hypodermal covering is formed in place of the 

 lost epidermis. 



The cortex may be divided into five layers : (i) the single hypodermal 

 layer ; (2) exo-cortex, a layer of from one to three or four tiers of closely 

 packed cells without intercellular spaces ; (3) medio-cortex, a tissue 

 composed chiefly of large air-canals and constituting nine-tenths of the 

 bulk of the root ; (4) endo-cortex, consisting of one or two layers of cells 

 closely set together, lying adjacent to (5) the endodermis (Figs. 8, 9). 



The first of these layers has already been described. The exo-cortex 

 is continuous, between the air-canals, with the medio-cortex. In N. 

 odorata (Fig. 8) there are between the outermost air-canals and the 

 hypodermis two series of cortical cells with thin walls and in size of lumen 

 equal to or larger than the hypodermal cells. N. tuberosa and N. 

 marliacea-chromatella have usually three series of cells in the exo-cortex ; 

 they are rounded, with collenchymatous walls, and larger by half than the 

 hypodermal cells. The exo-cortex in N. lotus consists of a single row 

 of cells (four or five near the base of a mature root), in N. flava of one or 

 two rows. In N. elegans x zanzibariensis there are two or three series of 

 cells which are thin-walled and three or four times as large in cross- 

 section as the hypodermal cells ; in longitudinal section, however, the 

 hypodermal cells are much the longer, being about 0.025 mm. long ; the 

 exo-cortical cells are only 0.0175 mm. in length, and the cells of the medio- 

 cortex about 0.0076 mm. N. flavo-virens, at least near the proximal end 

 of the root, has five or six cell-layers in the exo-cortex. 



The outstanding characteristic of the medio-cortex is the presence of 

 large air-canals. These are elliptic or rounded or more or less hexagonal 

 in cross-section, and run parallel throughout the length of the root, 



