MORPHOLOGY 153 



also the vascular bundles and accompanying tissues of the central 

 cylinder. The terms dermatogen, periblem, and plerome are em- 

 ployed merely for convenience to designate certain cell layers, and 

 are not to be regarded as significant of any peculiar histogenetic or 

 tissue-forming ability. The external layer from which the epidermis 

 develops usually remains a single cell layer. The rudiments of the 

 still undeveloped vascular bundles soon appear in the central cylinder 

 as procambium strands ; while the endodermis of roots is derived at 

 an early stage from the innermost layer of the cortex. 



In stems with apical cells the rudiments of new leaves and 

 shoots are developed from single peripheral cells, or cell groups of the 

 vegetative cone (Fig. 162). In such cases, not only the new shoots, 

 but even the leaves, usually begin their development with an apical 

 cell. The apical cells of the leaves, however, soon disappear, and 

 further growth proceeds along their whole margin. 



In a stem with no apical cell (Figs. 16, 164) the rudiments of the 

 leaves and new shoots first appear as small protuberances, the formation 

 of which is generally initiated by the periclinal division of a group 

 of periblem cells ; while, in the meantime, the cells of the overlying 

 dermatogen continue their characteristic anticlinal divisions. In the 

 case of new shoots developing at some distance from the growing 

 point of the parent stem, the cells from which they are destined to 

 arise retain for that purpose their original embryonic character. In 

 spring, as Ludwig Koch has shown, the formation of the buds on the 

 rapidly growing shoots of bushes and trees may be postponed, so 

 that the rudimentary lateral shoots first appear in the axils of the 

 eighth or even the tenth youngest leaves, and consequently at points 

 where the differentiation of the surrounding tissue has already begun. 



The vegetative cone, in the case of strictly dichotomously-branch 

 ing shoots (cf. Fig. 14), increases the number of its cells in the 

 direction of the plane of the subsequent bifurcation, and eventually 

 gives rise to two new growing points. With the exception of those 

 Pteridophytes, whose roots as well as stems are dichotomously branched, 

 the branches of the roots arise in acropetal succession ; and their 

 branching first begins in regions considerably removed from the grow- 

 ing point, and where the differentiation of the tissues is already 

 complete. In Phanerogams new roots are developed in the pericycle : 

 in Pteridophytes in the innermost cortical layer. The lateral roots 

 must consequently push through the whole cortical layer of the parent 

 root. They are situated either directly in front of the vascular strands 

 of the parent root, or between the xylem and phloem strands. The 

 number of rows of lateral roots is, therefore, as Van Tieghem has 

 pointed out, either equal to or double the number of vascular strands. 

 As the strands of the vascular bundles of roots take a straight course, 

 the lateral roots must similarly form straight rows. The distances 

 between the rows themselves are equal, or when the lateral roots are 



