DIATOMS 



207 



of almost pure silica slowly accumulate at the bottom of the 

 water. Instances are afforded by the extensive beds of " dia- 

 tomaceous earth " found at Dolgelly in Wales, at Bilin in Bohemia, 

 and elsewhere, some being of marine, others of freshwater origin. 

 Such deposits are utilised commercially in the preparation of 

 dentifrices and, owing to the very small size of the individual 

 particles, for mixing with nitroglycerine in the manufacture of 

 dynamite. The extensive 

 vegetable Plankton of the 

 sea at times consists almost 

 entirely of Diatoms. 



Although several groups 

 of Algae are designated ac- 

 cording to the prevailing 

 colour of their chloroplasts, 

 they are characterised by 

 many more important 

 features, and especially by 

 the nature of their repro- 

 ductive processes (see next 

 chapter). The chloroplasts, 

 in general, assume the most 

 complex forms among the 

 Green Algae, where there is 

 often but a single one in 

 each cell, a condition al- 

 ready noticed in Chlamy- 

 domonas and its allies (cf. 

 p. 181). Moreover, the 

 chloroplasts of the Green 



Algae commonly possess one or more pyrenoids and, during active 

 assimilation, starch is formed as a reserve-product, first around 

 the pyrenoids, and then in the general substance of the plastid. 

 Whilst most of the colonial (e.g. Scenedesmus) and less differ- 

 entiated filamentous forms (e.g. Ulothrix) have a relatively 

 simple undivided chloroplast, greater complexity is met with 

 in many of the more highly organised Green Algae. 



The chloroplast of Ulothrix is a curved band having the form 

 of an incomplete cylinder (Fig. 115, a, c), which occupies the 



FIG. 113. Various Diatoms (only the 

 siliceous shells are shown), a and 

 c, Navicula ', b, Nitzschia ; d, Cym- 

 bella ; e, end-view, and /, side-view 

 of cells of the colonial Melosira. 



