Cytology and ultrastructure of the Xanthophyceae. II. The zoospore and vegetative cell of coccoid forms, with special reference toOphiocytium majusNaegeli

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Abstract
Zoospores and to a lesser extent vegetative cells of the coccoid xanthophycean algae Ophiocytium majus Naegeli, Mischococcus sphaerocephalus Vischer and Botrydiopsis alpina Vischer are compared with respect to their morphology and selected aspects of ultrastructure, by means of light microscopy and electron microscopy of shadowcast cells and sections. Observations in less detail indicate that zoospores of Bumilleriopsis filiformis Vischer, Pleurochloris meiringensis Vischer, Sphaerosorus composita Moewus, Chlorellidium tetrabotrys Vischer et Pascher and Chloridella neglecta Pascher have similar organisation to those of the three main species. Chloroplasts of Bumilleriopsis filiformis lack a girdle lamella, only the second record of this in heterokont algae. Pyrenoids are reported for Pleurochloris meiringensis, Bumilleriopsis filiformis and Mischococcus sphaerocephalus, for the first time in the latter two species. Other observations on chloroplast structure and on the flagella, eyespot, flagellar swelling and Golgi bodies emphasise the striking similarity between the zoospores of these coccoid Xanthophyceae and of filamentous species described previously. It is suggested that the Xanthophyceae sensu stricto forms a natural group with other heterokont classes, i.e. the Chrysophyceae sensu Christensen (excluding the Haptophyceae), the Phaeophyceae and the Bacillariophyceae.