FTERIDOPHYTA 



261 



and in some of the Tree-ferns form a thick matted mass completely 

 covering the stem. The roots arise eiidogenously, the apical cell 

 being derived from a cell of the endodermis of the vascular bundle 

 of the stem, and the root finally breaks through the overlying tissues 

 of the stem and leaf-base. 



Secondary roots are found in most Ferns, developed laterally upon 

 the larger roots. These rootlets (Fig. 226) always arise from a 

 special rhizogenic cell, which is an endodermal cell opposite the 

 primary xylem. When the roots are diarch, as in the Polypodi- 

 aceee, there are, therefore, two 

 rows of lateral roots devel- 

 oped. The rhizogenic cells 

 are especially conspicuous in 

 Marsilia. In the rhizogenic 

 cell three intersecting walls, 

 enclosing a tetrahedral cell, 

 are developed, and the latter 

 at once becomes the apical 

 cell of the new root. The 

 basal cells, formed by the first 

 three walls, form a sort of 

 pedicel connecting the rootlet 

 with the vascular bundle of 

 the main root, with which its 

 tissues are continuous. The 

 layer of cells immediately sur- 

 rounding the end of the young 

 root form what has been 

 called a "digestive pouch" 

 (Fig. 226). 



The segmentation of the apical cell of the root in the typical 

 Ferns is extremely constant. Segments are cut off in regular suc- 

 cession from the lateral faces, and corresponding to each series of 

 lateral segments there is one cut off from the outer face, which con- 

 tributes to the root-cap (Fig. 225). Each lateral segment is first 

 divided by a nearly radial wall into two, so that a cross-section back 

 of the root-apex shows six radially arranged cells, three of which 

 do not extend quite to the centre. Periclinal divisions next separate 

 a central group of cells which gives rise to the central stele, or vas- 

 cular cylinder. The outer cells later become separated into the cortex 

 and epidermis. 



The cap segments divide first by intersecting vertical walls into four 

 cells, which undergo repeated divisions and form the regular layers 

 of the root-cap. Each layer of cells divides once by periclinal walls, 

 so that two layers of cells arise from each primary cap segment. 



FIG. 226. Pteris Crelica. Origin of lateral 

 rootlet from the endodermis of the root 

 (after VAN TIEQHEM) ; en, endodermis of 

 root ; x, apical cell of rootlet ; p, " digestive 

 pouch." 



