Observations on the Cytology and Ultrastructure of the New Algal Class, Eustigmatophyceae

The zoospores and, to a lesser extent, the vegetative cells of six species of Eustigmatophyceae have been investigated by means of light and electron microscopy. Five species have lageniform zoospores with a single flagellum bearing a bilateral array of stiff hairs; this is directed anteriorly during swimming; a short basal body with no emergent axis indicates the basically biflagellate condition in these species. The sixth has the second flagellum developed as an emergent organelle; this is smooth and short with a long hair-point and is always directed laterally or posteriorly during swimming. A swelling is borne at the proximal end of the anteriorly directed hairy flagellum, even in the biflagellate species. The swelling consists of an expansion, Tshaped in cross section and layered internally, of the dilated flagellar membrane. The swelling is always closely applied to the cell membrane in the region of the eyespot. The eyespot is a large conspicuous orange-red body in the extreme anterior end of the motile cell; it is extraplastidic and consists of an irregular group of droplets, not bounded by a membrane either collectively or individually. The zoospores contain a single elongate chloroplast and the vegetative cells a much-lobed parietal one. The chloroplast in both the motile and vegetative cell contains three-thylakoid lamellae; endoplasmic reticulum bounds the chloroplast but is not in continuity with the nuclear envelope. Pyrenoids are never present in the zoospores but the vegetative cell always contains a unique form of pyrenoid; this is polygonal, projects from the inner face of the lobed chloroplast and is connected to the latter by a very narrow stalk. It is surrounded by flat plates of an unidentified photosynthate which appears finely lamellate after fixation. Thylakoids do not enter the pyrenoid matrix. The nucleus lies between the eyespot and the anterior end of the chloroplast in the zoospores; it is pyriform in shape (though not markedly elongate) with the anterior end extended towards the flagellar basal body. Golgi bodies (as recognizable stacks of cisternae) are absent from the zoospore of the uniflagellate species but present in the vegetative cell. These features of organization are compared in detail with the same components in the Xanthophyceae and other classes of algae, with particular reference to the motile cells. It is concluded that the Eustigmatophyceae is less like the Xanthophyceae sensu stricto than the latter is like t