132 PART II. ANATOMY AND HISTOLOGY. [ 34 



be traced quite to the extremity ; they disappear further back, 

 and their place is taken by parenchymatous cells. 



Bundles often terminate in connection with glandular Jissue ; 

 for~instance, in chalk-glands and nectaries. 



34. Histology of the Development of Secondary 

 Members. It has been already pointed out (p. 9) that the 

 growing-point is the seat of development, not only of new tissue, 

 but also of new members ; and further (p. 18), that secondary 

 members are developed either by dichotomy or by lateral out- 

 growth. 



A. Development of normal branches of the shoot_Q3L_Q_the 

 thallus only takes place at the growing-point. 



a. By dichotomy. Two modes may be distinguished accordingly 

 as the growing-point has or has not an apical .cell : 



when there is an apical cell, true dichotomous branching is 



A 



Vio. 108. A B C successive stages in true dichotomons branching by longitudinal 

 division of an apical cell; from the shoot of an Alga Dictyofa dichotoma (highly magni- 

 fied; after Naegeli). 



effected by the longitudinal division of the apical cell into two, 

 each of which becomes the apical cell of a branch : 



when there is no apical cell, the growing-point becomes 

 broadened, and the central portion of it passes over into condition 

 of permanent tissue, leaving two distinct masses of embryonic 

 tissue, which constitute the growing-points of the two branches 

 (e.g. Marchantiaceae). 



b. By lateral outgrowth : 



irlii-n 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 Algse, or more commonly from a seg- 

 ment of the initial cell, as in many Algse, 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- 



