Читать книгу The Diatomaceæ of Philadelphia and Vicinity - Charles S. Boyer - Страница 13

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Unicellular or filamentous. Cells either free, sessile, united in filaments, immersed in a gelatinous envelope or in fronds composed of branching tubes; microscopic, enclosed in a more or less siliceous envelope (frustule), composed of two parts (valves), usually connected by an intervening band (zone or girdle). Cell contents include yellowish or brownish chlorophyll-like bodies which occur in one or several bands (placcochromatic), or as variously distributed granular masses (coccochromatic) lining the inner walls. Growth by ordinary cell division or by auxospores; sexual multiplication by the formation of sporangia. Valves of two kinds: (a) Those in which the markings or parts are more or less concentric (Centricæ); (b) Those (Pennatæ) in which the parts are more or less symmetrically divided by a line (pseudoraphe) or by a cleft (raphe).