PRACTICAL BOTANY 



1. The cylindrical form of the thallus, and the slight inequality 

 of the apparently dichotomous branching. 



2. The general structure of the mature parts of the thallus, 

 consisting of 



a. A series of large central cells, with dark reddish-brown 

 contents : these are surrounded by 



b. A single layer of pericentral or cortical cells, which are 

 arranged with considerable regularity in rings, each ring 

 corresponding to, and surrounding one of the central cells : the 

 whole thallus is thus built up of successive tiers of cells. 



3. Observe also the apices of the branches, which taper off 

 to fine points, each terminating in a single dome-shaped apical 

 cell. 



Select a good specimen of an apex, and examine it in detail 

 under a high power : observe 



i. The conical ending of the branch, covered by a thick 

 mucilaginous wall, which extends backwards over the more 

 mature parts, and is covered externally by a definite and 

 continuous cuticle. 



ii. The single dome-shaped apical cell, with highly refractive 

 protoplasmic contents, and more or less obvious nucleus. 



iii. The successive segments which have been cut off from 

 its base by parallel transverse walls. 



iv. The subdivision of the segments by longitudinal walls, so 

 that each segment ultimately forms one of those tiers of cells of 

 which the whole thallus is built up. 



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

 note carefully under a high power the fine protoplasmic strands, 

 which extend through the swollen cell- wall, connecting the proto- 

 plasmic 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 : it may then be seen 

 in those sections which pass through the middle of one of the 

 tiers of cells above noted that 



