160 STRUCTURAL BOTANY 



The cells of Ulothrix are somewhat similar in structure 

 to those of our last type. Within the cell-wall is the 

 protoplasmic layer, or primordial utricle, in which the 

 nucleus is embedded. The chloroplast has the form of a 

 broad transverse band, in which pyrenoids are contained. 

 When, as is often the case, the cells remain short, the 

 chloroplast may occupy almost the whole length of the 

 cell, but when the latter grows longer the green band 

 forms a girdle around its middle part. In the interior 

 of the cell is a large vacuole containing only cell-sap. 

 When the filament is attached by one end, the root-cell 

 is nearly or quite colourless ; sometimes this organ of 

 attachment is branched, though the rest of the thread 

 always remains simple. The thallus grows throughout 

 its length, and has no special growing-point. The cells 

 divide by transverse walls to keep pace with the growth. 



2. KEPEODUCTION 



The reproductive cells of Ulothrix are active zoospores. 

 They are of two kinds, the most constant distinction con- 

 sisting in the number of the cilia, of which there are four 

 in the one kind and two in the other. Both vary much 

 in size, but on the average the quadriciliate is decidedly 

 larger than the biciliate form. We will first consider 

 the former. The larger zoospores arise from ordinary 

 cells of the filament ; each mother-cell may produce one, 

 two, four, or eight of these bodies from its contents. In 

 the first case the whole protoplasm (with the exception 

 of a thin external layer) is used up to form the single 

 zoospore ; if two are to be produced a transverse cell- 

 division first takes place, and each half of the contents con- 

 stitutes a zoospore ; if the number be four, a longitudinal 



