BOT.-VOL. I.] CAMPBELL— NAIAS AND ZANNICHELLIA. 9 



parts, but of course later undergoing periclinal divisions. 

 The plerome is well marked, and in longitudinal section 

 shows usually two rows of cells. A single large cell (fio-. 

 5, at), which is sometimes conspicuous, may perhaps be the 

 single initial cell for the plerome, but its position is such as 

 to make it also possible that it belongs to the periblem, and 

 that there are several initial cells for the plerome. 



In regard to the form of the stem-apex, N. jiexilis re- 

 sembles more nearly N. graminea, to judge from Schu- 

 mann's figure (fig. i), than it does JV. major (Magnus, 1870, 

 PI. IV, figs. 6-11), where the apex is shorter and thicker 

 than in any of the other species. The apex of the stem is 

 seldom perfectly straight, and the first indication of the 

 formation of the lateral appendages is the appearance of 

 two slightly projecting ridges, one immediately above the 

 other and placed upon the convex side of the apical cone 

 (%s. 3, 5, /^, /). The lower one of these is the leaf and 

 the upper the structure (primordium) which subsequently 

 gives rise to a lateral branch and a flower. A little later 

 there is found on the opposite side of the apical cone and 

 a httle higher up than the other leaf, another projecting 

 ridge, which does not, however, show any structures above 

 it. This is the second leaf of the pair (fig. 5, P). The 

 tissues of the appendages show the same arrangement as 

 those of the stem-apex and are continuous with them. 



The tissues of the mature stem are very simple, but their 

 origin is readily traceable to the primary tissues of the 

 apex. As in all aquatics, large air-channels are developed, 

 arising here between the two outer layers of the cortical 

 cells. In the specimen of JV. Jiexilis figured (fig. 6), there 

 were six of these intercellular spaces {i), and the general 

 structure is much the same as in JV. minor (Magnus, 1870, 

 PL VII, fig. 4). N. major (Magnus, 1870, PL VII, fig. 2) 

 has a more massive stem, a larger number of air-spaces, 

 and a thicker cortex. There is in all cases a single axial, vas- 

 cular bundle, with a well defined endodermis, showing, when 

 mature, the characteristic radial foldings of the walls, and 

 derived as usual from the innermost layer of the periblem. 



