Alcune Forme Teratologiche Comparse in Colture di Navicula Minima Grun. e Navicula Seminulum Grun

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Abstract
Some teratological forms present in cultures of «Navicula minima» Grun. and «Navicula seminulum» Grun. — The teratological frustules of the Navicula minima and of the Navicula seminulum which appeared in the old cultures on the agarized soil have examined at the electron microscope. The alterations are particularly concerned with the contour of the valve, the position, the size and the number of the central and polar nodules, the axial area often fragmented like the raphe and the striae are oriented variously on the plane of the valve. The causes of such alterations can be found in the nutritional conditions and the unfavorable environments. The examined teratological forms, however bizzar and numerous have in common the characteristic of presenting their structure in relation to the distribution of the 2ones of major thickness of the walls; which are the first to appear in the initial formation of the new thecae; for this reason it is in this moment that the definitive, normal or teratological structure of the frustules is outlined. Besides some considerations of taxonomical character are done.