380 PRACTICAL BOTANY 



with sea-water, be hardened in successively stronger 

 alcohol from 40 to 90 per cent. 



From material thus treated, select a thallus which 

 does not apparently bear any of the reproductive organs 

 above mentioned ; mount a piece of it, including the 

 tips of some of the branches, in 50 per cent, glycerine 

 and water, and observe under a low power 



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 elongated 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. 



