The morphology and microanatomy of Platymonas impellucida sp.nov. have been investigated using light and electron microscopes and unusual stages of cell division studied. The anatomical features closely resemble those recently described for Platymonas convolutae (cf. Parke & Manton, 1967). As in that species the pyrenoid is penetrated from many directions by cytoplasmic canaliculi lined with the double plastid membrane but unlike that species a starch shell is not developed around the pyrenoid. In whole cells of P. impellucida therefore, it is extremely difficult, in most cases impossible, to detect the presence of the pyrenoid. This new information on the absence of a starch shell in a member of the Prasinophyceae is of importance since pyrenoid structure appears to be of major importance for taxonomic purposes. Apparent fusion stages have been shown to be a variation in vegetative division not previously encountered in the genus Platymonas.
Introduction
Recent studies on members of the Prasinophyceae have assisted greatly in characterizing this class of green-pigmented algae although the limits are not as yet entirely defined. Fine-structure investigations have, in addition, elucidated certain features of generic significance. The genus Platymonas is now well described and can be unequivocally distinguished from Prasinocladus, a closely related genus. Previously these two genera were separated on the presence or absence of non-motile dendroid colonies, but such colonies are now known in both genera (Parke & Manton, 1967).
The remaining significant diagnostic generic characteristic is the structure of the pyrenoid. In Platymonas the branched canaliculi invading the pyrenoid either from a direction facing the nucleus (Gibbs, 1962; Manton & Parke, 1965) or from many directions (Parke & Manton, 1967) contain cytoplasm rather than protrusions of the nucleus as in Prasinocladus (Parke & Manton, 1965).
