422 PRACTICAL BOTANY 



seen transverse striae : these are indications of past 

 cell-divisions. 



Examine the filaments under a high power, and pay 

 special attention to these striae and other irregularities 

 of the otherwise smooth cell-walls. It will then be 

 seen that the striaa are small, sharp-edged, ring-like 

 projections on the outer surface of the wall : also that 

 a single corresponding stria is to be found, more or less 

 distinctly marked, at the base of each cell. 



In some cells there may be seen an annular in- 

 growth of the cell-wall immediately below the 

 stria3 : note its form and connexion with the cell-wall ; 

 also, when seen in optical section, a central, dark mark : 

 it is here that the ring splits, and by stretching of the 

 ring the well-known process of intercalation of a new 

 zone of cell-wall follows. Examine actively-growing 

 filaments, and try to observe various stages of this 

 peculiar process, noting also any indications of cell- 

 division which follows it, the new septum being 

 formed immediately below the thin-walled intercalated 

 zone. 



Passing to the examination of the cell-contents, 

 observe 



1. The colourless protoplasmic basis, in which are 

 embedded 



2. The chromatophores, which appear as elongated 

 and branched rod-like bodies, more or less closely and 

 irregularly connected together : here and there will 

 be seen highly refractive pyrenoids attached to the 

 chromatophores : these are clearly to be distinguished 

 by their dusky purple staining on treatment with 

 iodine solution. 



