. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . s im-perfect transverse septa which pass inwards from the valves. Thus the interior of the diatom-cell may become chambered in variousways, and as will be noted subsequently these chambers very largelyaccommodate lobes of the chromatophores. SYMMETRY OF THE DIATOM-CELL. The diatom-cell may be perfectlysymmetrical or it may be completely asymmetrical, and the various degrees External > •// 87 of symmetry or asymmetry have been brought about
. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . s im-perfect transverse septa which pass inwards from the valves. Thus the interior of the diatom-cell may become chambered in variousways, and as will be noted subsequently these chambers very largelyaccommodate lobes of the chromatophores. SYMMETRY OF THE DIATOM-CELL. The diatom-cell may be perfectlysymmetrical or it may be completely asymmetrical, and the various degrees External > •// 87 of symmetry or asymmetry have been brought about by unequal growth ofeither the valves or the girdle. This question of symmetry was regardedas of primary importance from a classificatory point of view by both Heibergand Pfitzer, but unless taken in conjunction with other characteristics it is oflittle real importance. 0. Mtiller (95) has given an excellent synopsis of the different kindsof external symmetry observed in various types of diatoms. Some arezygomorphic in one plane only, some in three planes at right-angles, andothers exhibit a radial symmetry (fig. 60). In the majority of diatoms. Fig. 60. Diagrams to illustrate the symmetry of the diatom-cell. A, Navicula viridis; B,Gomphonema elegans; C, Amphora ovalis ; D, Eupodiscns Argus ; E, Isthmia enervis. A—Care pennate diatoms, D and E are centric diatoms, v, valve view ; <;, girdle view ; s, sectionof cell; pp, plane of section. (All the figures somewhat modified from O. Miiller.) the sculptures on the valves are also arranged in relation to the externalsymmetry, 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 g
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