206 



DIATOMS 



of variously shaped gelatinous clumps (Fig. 112, B) in which 

 are embedded numerous tortuous chains of rounded cells, inter- 

 rupted here and there by slightly larger colourless ones (heterocysts , 

 seen also in Tolypothrix, Fig. 112, E, h.) of uncertain function. 

 The Cyanophyceae often play a very important part in nature as 

 the first plants to colonise bare ground. 



In every habitat where other Algae occur there can usually 

 be found unicellular, or more rarely colonial, forms known as 

 Diatoms (Fig. 113) which possess quite special characteristics of 



FIG. 112. Diverse Blue-green Algae. A, Chroococcus. B, Single thread 

 from colony of Nostoc (shown natural size in &). C, Glceocapsa. 

 D, A nabcsna (with spore, Sp.). E., Tolypothrix. F.Lyngbya. G, Rivu- 

 laria. h., heterocysts ; sh., sheath. 



their own, and whose exact relationship to the other groups is not 

 clear. The individuals are either unattached, and in that case often 

 endowed with a power of fairly rapid movement, or else fixed to 

 the surface of larger Algae and other aquatics. The Diatoms 

 are brown or greenish in colour, though a few species are colour- 

 less and saprophytic, and each plant is provided with a cell-wall 

 richly impregnated with silica, and usually bearing a symmetrical 

 and often highly elaborate pattern of sculptured markings (Fig. 

 113). With the death of the organisms the practically unaltered 

 siliceous shells sink, so that, where Diatoms are plentiful, deposits 



