400 PKACTICAL BOTANY 



one from the base. The lateral segments undergo further 

 divisions according to the type of the limiting layer ; the basal 

 segments divide up, and ultimately pass over into medullary tissue. 

 If sections through the apex be stained with hsematoxylin, it 

 may be observed that each cell contains a single nucleus : this 

 observation may also be extended to the tissues of the older parts 

 of the thallus. 



VII. Cut successive transverse sections of the apex, 

 so as to pass immediately below the base of the de- 

 pression : one of these sections will include the initial 

 group, which will then appear to be composed of some 

 four or five cells, of oblong form, placed in a row, 

 side by side. 



Examining these under a high power it may be seen that seg- 

 ments are cut off (1) from either side of their oblong outline, (2) 

 from the sides of those of the initial cells which are at either end 

 of the row, and (3) that any of the initial cells may divide by a 

 median wall into two similar cells : by the latter process the 

 number of initial cells may be increased, and this is usually the 

 case before dichotomy : this is brought about by the cells at the 

 middle of the row ceasing to act as initial cells, and passing over 

 into the condition of permanent tissue, while those at either end 

 of the row retain the character of initial cells, and are to be re- 

 cognized as the two independent initial groups of the new 

 branches : thus the branching is a dichotomy. The attempt 

 should be made to find sections illustrating this process of 

 branching. 



This type of apex is by no means constant for the Fucoids ; 

 thus in Himanthalia there has been found in each branch of the 

 thallus only a single apical cell, which appears triangular in 

 transverse section, but in longitudinal section it shows two con- 

 vex sides : the form of the cell is thus, roughly speaking, that of 

 a Brazil nut. 



VIII. Cut transverse sections through the fertile 

 branches of the thallus, so as to traverse the mature 



