186 PART II. THE INTIMATE STRUCTURE OF PLANTS. [ 34. 



The primary meristem of the leaf becomes differentiated into 

 tissue-systems corresponding to, and continuous with, those of the 

 stem which bears it. In the developing leaves of those vascular 

 plants which have common bundles (see p. 177), the differentia- 

 tion of the protoxyleni begins at the point of junction of leaf and 

 stem, extending outwards in the procambium-strands of the leaf, 

 and inwards in those of the stem. 



The development of secondary branches of the leaf takes place 

 in essentially the same manner as that of the leaf from the stem. 

 Dichotomous branching of the leaf (see p. 51) takes place in the 

 same way as dichotomous branching of the stem. 



It will be seen that the development of a leaf on any stem takes 

 place in essentially the same way as the development of a lateral 

 branch on that stem ; it is only later that leaves and branches 

 assume their distinctive characters. 



C. Development of Branches of the Root. It has been pointed out 

 that the only normal secondary members produced by the root 

 are root-branches, or secondary roots ; these may be developed 

 either by dichotomy or by lateral outgrowth. 



a. By dichotomy. This has only been observed in certain spor- 

 phytes among the Pteridophyta (Lycopodium, Isoetes). Here the 

 growing-point broadens, under the root-cap, the central portion 

 passing over into permanent tissue, whilst the two sides remain 

 merismatic and form the growing-points of the two secondary 

 roots ; the old root-cap is exfoliated, and each growing-point forms 

 a new one for itself. The successive dichotomies take place in 

 planes at right angles to each other. 



b. By lateral outgrowth. It has been already stated (p. 62) that 

 the lateral development of secondary members does not take 

 place at the growing-point of the root, but at a considerable 

 distance behind it, where the tissues have already assumed 

 their permanent differentiation. The lateral roots are developed 

 endogenously from a layer of this tissue which remains embryonic 

 longer than the adjacent tissues. This layer may be either the 

 pericycle, as in Phanerogams, or the endodermis, as in most Vascular 

 Cryptogams. 



In the Phanerogams (Fig. 142), the growing-point of a lateral 

 root is formed by the growth and division of a group of pericycle- 

 cells, lying usually just externally to the outer end of a xylem- 

 bundle ; hence there are as many longitudinal rows of lateral roots 

 produced as there are xylem-bundles in the parent root, and cor- 



