34] 



CHAPTER II. THE TISSUES. 



185 



when there is a single initial cell in the growing-point, the 

 growing-point of the branch is developed either directly from the 

 initial cell itself, as in some Algae (Fig. 141 A), or more com- 

 monly from a segment of the initial cell, as in many Algae {Fig. 

 141 (7), Mosses, Liverworts, etc. : 



when there is not a single initial cell (e.g. Phanerogams), the 

 growing-point of the branch is formed by division of cells of the 

 periblem, including several layers, which grow and divide, form- 

 ing a lateral protuberance with the growth of which the dermato- 

 gen keeps pace ; the primary meristem of the branch undergoes 

 differentiation into tissue-systems corresponding to those of the 

 parent members, and continuous with them. 



B. 



C. 



FIG. 141. Illustrating development of lateral members; A (Stypocaulon scoparium, x30): 

 from the apical cell itself : B (Desmarestia ligulata, x 60) from the segments of an inter- 

 calary growing-point ; C (Chcetopteris plumosa, x 40) from the segments of the apical cell. 

 (After Falkenberg). 



Normal branches, however the details of their development may 

 vary, agree in this, that they are, with rare exceptions (see p. 

 20), of exogenous origin. 



B. Development of Leaves only takes place at the growing-point 

 of a stem, and always by lateral outgrowth (see p. 45). 



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 

 itself, or, more commonly, from the whole or a part of a segment 

 of the apical cell. 



When the growing-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. 



