The diatom flora of the mineralogenous acidotrophic inland waters. of Japan consists of 12 forms of diatom, among them Pinnularia Braunii var. amphicephala, Pinnularia acoricola, and Eunotia septentrionalis are most important because of their wide distribution and abundant reproduction.
It was proved by SEM observations that the valvocopula of the diatom Denticula tenuis var.crassula was composed of three cingular elements, namely bisegmented valvocopulae having traces of septa and septum-like plate, and some crossbars. It seems to be the most probable based on the microstructure of valve and girdle that the genus Denticula seas. str. is a heterogeneous taxon including at least two ancestries. The one is the group distributed in Sumatra and its vicinities having the constructions, for instance raphe system, pore occlusion and valvocopula etc., in common with the genus Epithemia BRÉB.. The other is the group which is close related with the section Fragilariopsis of Nitzschia HASSALL and Nitzschia valdestriata AL. et HUST..
Many anomalous cells of Nitzschia pungens, bearing 1-3 swellings in the girdle, were found in wild samples collected from eutrophic coastal waters. The swellings appear as extrusions of the girdle. They are accompanied with undulation of the valve margin and sometimes with a slight disorder of the interstriae. The cause is unknown but it is presumable that some organic compound in drainage from the land might be responsible for causing deformed cells.
The present research pursues to discuss the origin and evolution of freshwater diatoms based on the data obtained by the surveys for the inorganic acid lakes in Japan and alkaline lakes in Africa. From this research, next two ideas were assumed. 1. The families included many acidophilous and acidobiontic taxa appeared at early phase in the process of diatom evolution, whereas these included man y alkaliphilous and alkalibiontic taxa appeared at late phase in both the ce ntric diatoms and the pennate. 2. Freshwater diatoms appared in earlier phase of the Earth's history than the marine.
The micrographs in this report are from diatom material taken from the pond in Shiroganedai, Tokyo. Valve elliptical with parallel to slightly convex sides; ends capitate to rostrate. From the various degree of protrusion ends, it -would be better to consider Navicula bryophila var. capitata to the synonym of the nominate variety. When this species reduces its size, the shape becomes simple since smaller individuals often take weakly protruded ends. Axial area narrow, lineate. Central area sligh tly wider than the axial area usually. Striae radiate, convergent at the ends and alternately longer and shorter at the center of the valve. Length,12.5-23.0μm and the mode is 18and 19μ am which coincide with the range 10-25μm reported already. Breadth,4-5, am and the mode is 4.5μ am which show no difference previously reported range 2.5-5.0μm. Striae,29-32 in 10μm with the mode of 28 in 10μm which is in accord with the range 20-40 reported until now. The striae at the both ends are denser than those about the central area. We observed many individuals with triundulate margins of valves. Other morphological characteristics of these individuals were almost the same as Navicula bryophila. Length,12.5-18.0μm, and the mode is 16 and 17μm. Breadth,3-4μm and mode is 3.3μm. Striae,28-32 in 10μm and the mode is 30. So, the valves with triundulate margins are smaller than those of Navicula bryophila. This type was first illustlated by KOBAYASI (1960) an d later related to Navicula bryophila by ANDO (1979). Though this species is somewhat similar to N. rautenbachiae CHOLNOKY, it is distinguished by the narrower breadth and the coarser striae at the central area than those at both ends. In our observation it is better to distinguish this type form N. bryophila.