Prasiolales (Chlorophyta) von Helgoland

In their natural habitats, some representatives of Prasiolales are regularly, or often, washed by waves; others are sprayed with sea-water only occasionally, or not at all. In correspondance to this ecological grouping, species require marine or freshwater media respectively for optimal growth in cultures. The halophilic group consists ofPrasiola stipitata andRosenvingiella constricta. Though not analysed in detail, morphological characters and behaviour in cultures suggest that several different species are united under the namePrasiola stipitata. Rosenvingiella constricta, new for Helgoland and thus far reported only twice in Europe, is a dioecious haplont; both meiosis and genotypic sex-determination occur in the germinating zygote. The parenchymatous “Gayella-stage” ofRosenvingiella polyrhiza proved to be male and female gametangia, producing spermatozoids and non-motile eggs respectively. Fertile threads were extremely rare in natural material and — contrary toRosenvingiella constricta — did not occur in cultures, so the life cycle could not be elucidated.Prasiola calophylla is frequently associated withRosenvingiella polyrhiza. The life cycle includes heteromorphic asexual generations, the spores of the leafy thallus giving rise to unicellular aplanosporangia.Prasiola furfuracea reproduces only by spores. Samples ofRosenvingiella polyrhiza frequently containPrasiolopsis in addition to a number of other Chlorophyceae. Consequently, it is easy to appreciate that previous investigators came to erroneous conclusions about life histories and biological relations in studies based only on field material.