382 



MANUAL OF BOTANY 



outer one divides again into two, which are the initial layers 

 of the peribleni and dermatogen respectively. The little mass 

 continues to grow, becoming conical, and gradually working its 

 way to the exterior, absorbing by a kind of digesti^-e process the 

 cells of the cortex of the main root as it advances. 



If the stele contains more than two xylem bundles, a row of 

 lateral rootlets usually arises at the back of each. If only two 

 are present there are generally four rows of rootlets which are 

 placed in pairs, one a little on either side of each xylem mass. 

 In the Grasses and a few other Monocotyledons they arise 

 opposite to the phloem bundles, as they do also in some 

 Dicotyledons, where a resin-duct lies in the pericyole behind 

 each xylem bundle. 



In most Vascular Cryptogams the meristem of the apex is 



characterised by the 

 l'i«. ibj. presence of an apical 



cell (fig. 769, z.). This 

 has the shape of a four- 

 sided pyramid, the base 

 facing outwards. From 

 each face in sxiccession 

 segments are cut off, 

 which by subsequent 

 divisions give rise to 

 the tissue of the root. 

 After each division the 

 apical cell grows to 

 its original size before 

 cutting off the next 

 segment. The segment 

 parallel to the base of 

 the pyramid originates the root-cap, which is a many-layered 

 epidermis, as in the former cases mentioned. The dermatogen 

 in these plants does not produce any part of the covering of 

 the root, which is composed of the outer layer of the cortex. 

 The segments cut off from the other faces of the apical cell 

 divide by anticlinal and periclinal walls, formmg a mass of 

 tissue in which the limits of periblem and plerome soon become 

 apparent. The innermost layer of the former becomes the endo- 

 dermis, the outermost layer of the latter, the pericycle, except 

 in certain cases already mentioned, in which this layer is not 

 differentiated. Instead, the endodermis is composed of two 

 layers. 



Fig. 769. Longitudinal section tlirough root of 

 /^leris hastata, showing apical region, v. Apical 

 cell, from which are developed the tissue of the 

 substance of the root, o, c, and the root cap, 

 (f, i, wi, 71. After Sachs. 



