POLYSIPHONIA 381 



iv. The subdivision of the segments by longitudinal 

 walls, so that each segment ultimately forms one of 

 those tiers of cells (consisting of one central and 

 numerous pericentral cells) of which the whole thallus 

 is built up. 



Compare a number of apices in order to ascertain 

 the mode of branching : it will be seen that in some 

 apices the apical cell will have divided by an oblique 

 instead of a transverse wall : both of the two cells thus 

 formed gradually assume the properties of apical cells, 

 dividing in the usual way, and growing out right and 

 left as almost equal branches; but the inequality of 

 origin is still to be traced even in old branchings : it is 

 thus obvious that this is not a typical dichotomous 

 blanching, but a form of monopodial branching 

 closely allied to it. 



A perfectly typical example of dichotomy is to be found in 

 Dictyota dichotoma, and it may be easily observed on mounting 

 a piece of the thallus in weak glycerine, and examining under a 

 medium power. 



In mature parts of the thallus, as also near the young 

 apex, note carefully under a high power the fine proto- 

 plasmic strands, which extend through the swollen cell- 

 wall, connecting the protoplasmic body of the various 

 cells of the thallus one with another : for further 

 study of these recourse must be had to sections of the 

 thallus. 



III. Embed, and cut transverse sections of the thallus 

 of Polysiphonia, selecting such a part of it as is not too 

 old, i.e. about half-way between the apex and the base : 

 mount in glycerine and examine under a high power : 



