. A handbook of cryptogamic botany. Cryptogams. 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 als
. A handbook of cryptogamic botany. Cryptogams. 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, Fi
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