BOT.-VOL. I.] CAMPBELL— NAIAS AND ZANNICHELLIA. 



33 



which are all derivatives of the sister cells of the hypoderma, 

 from which they are finally separated by a well marked air- 

 space (fig. 77). From the innermost layer of the periblem 

 there is finally cut off a layer of narrow cells, in direct con- 

 tact with the plerome, forming the endodermis, or bundle- 

 sheath. 



A comparison of cross-sections of the root with a median 

 longitudinal section of the older embryo (see figs. 76, 78, 

 79) shows that the plerome is composed of a single axial row 

 of cells surrounded by a single layer of outer cells. The cen- 

 tral row can be easily traced to the single terminal cell of 

 the plerome, and in such sections as that shown in fig. 76 it 

 looks as if the lateral rows of cells might also be traced to 

 segments of the same initial cell. Whether this is always the 

 case it is not possible to say, but it is quite likely. A cross- 

 section of the root, a short distance back of the apex, shows 

 the arrangement of the cells plainly (fig. 78). The plerome 

 consists in this view of a single central cell surrounded by a 

 row of quite similar cells, and the whole complex is bounded 

 by the endodermis (en). A section of the same root higher 

 "P (%• 79) shows much the same arrangement of cells, but 

 the central cell of the young vascular bundle is larger now, 

 and in the ground-tissue of the root there have been devel- 

 oped numerous intercellular spaces (i). The central row of* 

 cells represents a vessel, but m none of the embryos exam- 

 ined was there seen any trace of the usual thickenings on 

 the walls. These cells early cease to divide by transverse 

 walls, and increase very much in length with the growth of 

 the root; but even in the oldest embryo examined there was 

 no sign of a breaking down of the division walls, and the 

 large elongated nucleus was conspicuous in each cell (fig. 

 74, a). 



The embryo grows rapidly and at the time the seed is 

 ripe fills up the embryo-sac completely. At maturity it 

 shows besides the cotyledon three other leaves; the first two 

 arranged almost opposite each other, the third forming a 

 slight angle with them (fig. 80). Between the cotyledon 

 and the enclosed bud there are two small scales (squamul^) 



