Ultrastructure of the scale-covered zoospores of the green alga Chaetosphaeridium, a possible ancestor of the higher plants and bryophytes

The zoospores of the green alga Chaetosphaeridium globosum are covered on all surfaces with tiny diamond-shaped scales similar to those of the prasinophycean flagellates and the Charales. The flagella also bear striated hairs (hair scales) so far considered to be a characteristic of the Prasinophyceae. The flagellar apparatus differs from that observed in the Prasinophyceae, shows many similarities to that of the Charales, and is identical with the “Vierergruppe” of the pteridophytes, cycads and bryophytes. The zoospores are opisthokont, with two flagella inserted subapically. There is a lateral chloroplast containing typical grana and intergranal lamellae, but no eyespot. The very complicated Golgi body/contractile vacuole system comprises 10–20 contractile vacuoles. A microbody occupies a characteristic position in the cell, and in a young germling contains a crystalline inclusion. The ultrastructure of the zoospore supports the old theory that the ancestors of the higher plants may well be found among Coleochaete and its relatives, past and present.