IX 



FILICINE^ LEPTOSPORANGIATJE 



337 



first- formed cap cell (or "epidermal segment," to use his termin- 

 ology), there is, in the central part, always a doubling of the 

 cells 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 pro- 

 cambium 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 sex- 



ea 



tants (Fig. 1 88, D) ; the 

 fourth arises from the in- 

 ner cell of one of the smal- 

 ler 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 com- 

 plete root are all indicated. 

 The bundle is bounded 

 externally by the endo- 

 dermis, w^hose cells are 

 much elongated trans- 

 versely, and clearly dis- 

 tinguishable from the peri- 

 cambium (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 

 (Fig. 1 88, E). The masses of procambial cells on either side 

 of this central line of cells constitute the young phloem. 



The primary tracheids (protoxylem) 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 



22 



Fig. 187. — Pteris cretica. Origin of lateral 

 rootlet from the endodermis of the root; en, 

 endodermis of the main root; x, apical cell 

 of the rootlet; p, "digestive pouch." (After 

 Van Tieghem.) 



