330 < MOSSES AND FERNS chap. 



by periclinal walls, so that each layer of the older root-cap is 

 normally double, except sometimes at the extreme edge. 



There is very little displacement of the cells for a long 

 time, and cross-sections of the root, made some distance below 

 the summit, still show the limits of the original sextant walls, 

 which form six radiating lines with periclinal walls arranged 

 with great regularity. In the centre the divisions proceed with 

 great rapidity, and the plerome soon shows the elongated 

 narrow procambium cells. In the centre are four much larger 

 cells, which develop later into tracheids, and three of these can 

 be traced back to the central cells of the three larger sextants 

 (Fig. 165, D) ; the fourth arises from the inner cell of one of 

 the smaller ones. This central group of cells marks the 

 position of the plate of tracheary tissue, found later in the 

 root. By this time the parts of the complete root are all in- 

 dicated. The bundle is bounded externally by the endodermis, 

 whose cells are much elongated transversely, and clearly dis- 

 tinguishable from the pericambium (pericycle), which consists of 

 one or two rows of cells. Inside this is the mass of procambium 

 cells, the large tracheids of the central part of the xylem being 

 very evident. The masses of procambial cells on either side of 

 this central line of cells constitute the young phloem. 



The primary tracheids (proto-xylem) arise simultaneously at 

 the foci of the section, and consist of a single line of narrow 

 pointed tracheids, with fine spiral markings, very closely set at 

 first, but later pulled apart somewhat with the increase in length 

 of the root. These are formed a long time before any other 

 permanent tissue elements can be distinguished. Around these 

 primary tracheids are formed a group of similar ones, and from 

 here the formation proceeds towards the central group of large 

 tracheids, which are the last to have their walls thickened and 

 lignified. The large secondary tracheids are scalariform, like 

 those of the stem. The cells of the pericycle remain nearly 

 unchanged, but in the two phloem masses, according to 

 Poirault,^ sieve-tubes are always present. These tubes are of 

 two types, with horizontal transverse walls, or inclined ones. 

 The perforations in the sieve-plates were demonstrated, and 

 lateral perforations, either isolated or in groups, also occur. 

 His statement that the sieve -tubes have no nuclei requires 

 further proof. The walls of the sieve-tubes are of cellulose, 



1 Poirault (l). 



