420 



PROTOPHYTA 



halves known as valves ; the sihcified cell-wall of the older of these 

 halves is slightly the larger of the two, fitting on to the younger one 

 like the lid of a cardboard box. The cell-wall is composed of an 

 organic matrix closely allied in composition to cellulose, impregnated 

 with silica or a compound of silica ; either of these two ingredients can 



be removed and the other left behind, 

 the former by calcination, the latter 

 by the action of hydrofluoric acid. In 

 those species which are fixed by a 

 gelatinous stalk, this stalk is also com- 

 posed of a substance allied to cellu- 

 lose. The overlapping edge of one of 

 the two valves over the other is called 

 the girdle or hoop ; this girdle may be 

 simple, or there may be several. In 

 many species — and probably in all, 

 if examined with a sufficiently high 

 power — each valve is marked with a 

 number of rows of very fine perfora- 

 tions, which, except under the very 

 highest microscopic powers, appear 

 as if confluent into strias or furrows. 

 There may be two or three sets of 

 these apparent stris, but they do not, 

 as a rule, reach to the centre of the 

 valve. So constant is the arrange- 

 ment and the fineness of these stria- 

 tions in some of the more abundant 

 species, that they furnish an admirable 

 test for the definition and angular 

 aperture of microscopic lenses. Some 

 species of Navicula (Bory) and Pleu- 

 rosigma (Sm.) are especially used for 

 this purpose. Some marine genera 

 in particular (Triceratium, Ehrb., 

 Coscinodiscus, Ehrb., &c.) are cha- 

 racterised by the beautiful honeycomb-like areolation of the cell-wall, 

 due to the presence in it of actual chambers, which rnay or may not be 

 covered by a thin membrane. The membrane at the bottom of these 

 chambers is also most minutely perforated, constituting what is known 

 as the secondary markings. In describing diatoms, the aspect in which 

 the girdle is turned towards the observer is spoken of as ^& front, girdle. 



Fig. 353. — Pinnularia viridis Sm. A , 

 valve-view ; 5, girdle-view (diagrammatic), 

 r, furrows ; ?«, raphe ; g, central node ; 

 >t, terminal nodules ; a, outer and older 

 valve ; /, inner valve : n, secondary' lines 

 ( X 800). (After Pfitzer.) 



