Observations on the morphology and oospore wall ornamentation in culture of the rediscovered Japanese endemic Nitella gracilens (Charales, Chlorophyta)

The Japanese endemic charalean species Nitella gracilens Morioka (= N. furc ata subsp. furcata var. sieberi f. gracilens) was thought to be extinct unt il it was rediscovered at the type locality, Lake Ashinoko, Kanagawa Prefec ture. Uni-charalean cultures of N. gracilens were established and sexual or gans developed and matured. The morphology of this alga, examined by light microscopy, corresponds closely with the original description by Morioka, p articularly with regard to the oospores. The mature oospore walls of N. gra cilens are orange or yellowish-brown and the fossa wall of the oospore is s mooth or has a finely granulate ornamentation. In addition, scanning electr onic microscopic examination demonstrated that the fossa walls have minute granulate structures that are compactly arranged These observations contras t with a previous report that the oospore wall ornamentation of this taxon is reticulate. Since oospore wall ornamentation and the form of the dactyls in N. gracilens are essentially different from those of other infraspecifi c taxa of N. furcata sensu Wood (1965), N. gracilens should be maintained a s a distinct morphological species within the genus Nitella.