. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . ternalsymmetry, a fact of such importance from a classificatory standpoint thatit allows the division of diatoms into the two well-marked primary groups ofthe Centricse and Pennatse. STRUCTURE OF THE CELL-WALL. The cell-wall of diatoms consists ofan organic ground substance which is more or less strongly silicified. Thegirdle is usually relatively thin, but the valves vary much in strength andthickness. In the great majority of species they a
. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . ternalsymmetry, a fact of such importance from a classificatory standpoint thatit allows the division of diatoms into the two well-marked primary groups ofthe Centricse and Pennatse. STRUCTURE OF THE CELL-WALL. The cell-wall of diatoms consists ofan organic ground substance which is more or less strongly silicified. Thegirdle is usually relatively thin, but the valves vary much in strength andthickness. In the great majority of species they are more or less sym-metrically sculptured, such markings generally having the appearance of 88 Bacillarieae striations or areolations. A few diatoms are known in which the valves aresmooth, no trace of striations or other markings having been diatoms, in which the wall is very thin and apparently structureless,are exemplified by Navicula perlepida, N. glaberrima, and Tropidoneisleevissima. Detailed microscopical investigation has shown that the markings consistof small cavities within the siliceous cell-wall which are in most cases. 0 Fig. 61. Triceratium Favus Ebrenb. A, valve view; B, transverse section across tbe rim of tbevalve ; C, girdle view ; D, surface view of portion of valve to sbow details of , limitations of broken part of valve to sbow tbe hexagonal chambers (ch) • o, externalopenings of the large chambers ; p, pore; po, poroids or dots ; s, spines at the corners of thehoneycomb-like chambers; w, external wing at the rim of the valve. (B after O. Muller ;C after W. Smith; D after Pfitzer.) arranged in regular rows, thus giving the appearance of striations. In somecases the marks are due to ridges, and both the cavities and ridges maybe either upon the inner or the outer side of the cell-wall. When thecavities giving rise to the appearance of striations are very small theyare known as punctse. The strife vary in strength from cons
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