34] CHAPTER II. THE TISSUES. 133 



gen. keeps pace ; the primary meristem of the branch vindergoes 

 differentiation into tissue-systems corresponding to those of the 

 parent members, and continuous with them. 



Normal branches, however the details of their development may 

 vary, agree in this, that they are, with rare exceptions^ exogenous 

 origin. 



B. Development of Leaves only takes placeji^the^rowing-poiut 

 of a stem, andjdways by lateral outgrowth (see p. 28). 



When the growing-point of the stem has a single initial cell, the 

 growing-point of the leaf is developed either from the apical cell 

 itsetf,l>r,'more commonly, from the whole or a part of a segment of 

 the apical cell. 



When the groicing-point of the stem has not a single initial cell, 

 as in Phanerogams, the growing-point of the leaf is formed by the 

 division of _cells belonging to one or more of the superficial layers 

 of the periblem, accompanied by growth and division of the cor- 

 responding cells of the dermatogen. 



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. 127), the differentiation 

 of the protoxylem begins at the point of junction of leaf and stem, 

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

 wards 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. 34) 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 sporo- 

 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 



