Eacillariece 



201 



Palmer and Keeley 1 have pointed out, each band of the girdle is a 

 two-ended band of silica with the ends overlapping without being 

 joined. Each frustule possesses a thin coat of mucus which can 

 be readily demonstrated by slight staining with aniline dyes. 



Diatoms often occur as solitary, free-floating individuals, but 

 they may adhere to one another to form chains, either by their 

 valve-faces to form ribbon-like or 

 thread-like colonies (Ewnotia, Melo- 

 sira), or by mucous cushions at 

 their angles to form zig-zag colonies 

 (Tabellaria). Some adhere closely 

 to larger plants by the whole of one 

 valve-face (Cocconeis), or they are 

 affixed to some larger object by 

 simple or branched gelatinous stalks 

 (Gomphonema); others occur in large 

 colonies embedded in a common 

 mucilaginous envelope, either as a 

 compact mass or a simple or branch- 

 ed tube. This condition is commoner 

 in marine species than in freshwater 

 ones, and is variable even in indi- 

 vidual species. Marine Diatoms 

 also reach a much larger size than 

 freshwater ones. 



In referring to any individual 

 Diatom, the aspect in which the 

 girdle side is exposed to view is 

 best termed the girdle-view (or 

 zonal-view), and that in which the 

 surface of the valve is exposed to 

 view the valve-view. 



The valves are as a rule thin and transparent, slightly convex 

 on the c.ftside, and in almost all species they are ornamented with 

 variously disposed strife. The best lenses, however, have shown 

 that these striae consist of series of small cavities within the 

 siliceous wall of the Diatom, and it is their close and regular 

 arrangement that causes them to appear as striae. The striae are 



IV 



Fig. 123. A, valve view of 

 Navicula nobilis Ehrenb. var. Dac- 

 ti/lnx (Ehrenb.) V. H., from Dol- 

 gelly, Wales ( x 250). B, girdle 

 view of N. major Kiitz., from 

 Shipley, W. Yorks. (x300). en, 

 central nodule ; iv, inner valve ; 

 ov, outer valve; pn, polar nodule; 

 r, raphe. 



Palmer and Keeley in Proc. Acad. Nat. Sci. Philadelphia, 1900. 



