CHAPTER XX. 

 DIATOMACEAE, PROTOCOCCUS, SCHIZOPHYCE^ (FISSION ALG^l). 



PRINCIPAL MATERIALS USED. 



Any large Diatom, e.g., Pinnularia (Navicula) viridis ; fresh. 



Protococcus viridis, from damp bark of trees ; fresh. Also Htxmatococcus 



pluvialis, from water-butt or roof-gutters. 

 Azolla caroliniana, for the Nostoc, Anafaena Azollce ; fresh. Or other 



Nostoc species. 



Oscillaria species, from standing water or wet soil ; fresh. 

 Gleocapsa species, from damp walls, rocks, or greenhouse pots or glass ; fresh. 



PRINCIPAL REAGENTS USED. 



Concentrated hydrochloric acid Concentrated sulphuric acid 20 per cent, 

 chromic acid Concentrated chromic acid Potassium permanganate 

 Calcined magnesia Iodine Chlorzinc iodine. 



Structure of Diatoms. The Diatomaceae, or Bacillariaceae, are 

 unicellular organisms which form a somewhat isolated group in 

 the organic world, showing their chief relationships, however, with 

 Desmideae, amongst Algae. The most favourable object upon 

 which to study the structure of the Diatomaceae is, perhaps, 

 Pinnularia (Navicula) viridis, a species very common in standing 

 and flowing fresh water. It is distinguished amongst fresh- water 

 forms by its comparatively large size, and allows in general an 

 easy insight into its structural relations. Under the microscope, 

 in which we must study them with the strongest objective at our 

 command, they appear either in the form of an elongated ellipse, 

 or as a rectangle with somewhat rounded angles. In the former 

 case, we see them from the side of the valve, test, or frustule 

 (Fig. 97, A), in the latter, of the girdle, or union of the valves (Fig. 

 97, B). We will call these the valve-side and girdle-side respec- 

 tively. On the valve-side, the cell-wall appears marked with 



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