INTRODUCTION. 



13 



for the peripheral part of the root-cap covering the sides of the apex of the root, 

 on the other for the dermatogen-layer of the root. The origin of these, i.e. the 

 lateral part of the root-cap, and of the dermatogen, takes place similarly to that of 

 the corresponding parts in the third type. 



II. In the Gymnosperms the differentiation of the meristem at the puncftwi 

 vegetaiionis of the root^ is essentially different from the types described for the 

 Angiosperms (Fig. 6). A plerome-cylinder with sharp contour occupies the centre 

 (/-/>). The longitudinal rows of cells which compose this converge at the rounded 

 apex towards a small initial group of cells. The plerome is surrounded by a mantle of 

 periblem consisting of many (e.g. 

 in Thuja occidentalis of 12-14) 

 concentric layers arranged with 

 considerable regularity. Each one 

 of the inner layers (in Thuja, 8-10) 

 of this mantle has its initial group 

 above the apex of the plerome. 

 The division of these cells per- 

 pendicularly to the surface (i.e. ra- 

 dially) brings about the increase of 

 the surface-elements of the layer. 

 At the same time successive divi- 

 sions parallel to the surface, that 

 is, a doubling of the layers, takes 

 place in the apical region (Fig. 

 6, {). Since the radial walls of 

 the successive layers fit almost 

 exactly one on another, the cells 

 of the periblem mantle are ar- 

 ranged above the apex in cor- 

 respondingly regular rows. As 

 the layers are pushed outwards 

 above the apex by their succes- 

 sive doubling, division ceases in 

 them, and increase of volume of 

 the cells takes place; those which 

 happen to be outermost at the 

 apex become gradually loosened, 

 and pushed off as a root-cap. Here then it is not possible to distinguish a layer of 

 calyptrogen or of dermatogen ; the outermost periblem acts as root-cap covering the 

 meristematic apex. The radially arranged apical prolongation of the periblem is in 

 all cases relatively strongly developed, its height is usually equal to or greater than the 

 whole diameter of the root, rarely (Taxus, Cycas circinalis) it is smaller. According 

 as it is more strongly developed, the arrangement of the cells in rows is more clearly 

 apparent ; e. g. Pinus, Ephedra, Zamia integrifolia. 



Fig. 6. — {190) yunitc:. - Oxycedrus. Median longitudinal section through 

 the apex of a lateral ro • p—p plerome, surrounded by about sixteen layers 

 of periblem, the outermost of which represent the root-cap ; i the initial region 

 for periblem and plerome. 



' Strasbui-ger, Die Coniferen, &c., p. 340.— Reinke, Morpholog. Abhandl. p. i. — Janczewski, I.e. 



