THE GENUSCRATICULA

Auxospore formation, reproductive plasticity and cell structure inNavicula ulvaceaand the resurrection of the genusDickieia(Bacillariophyta)

“…unlike Craticula (Mann & Stickle, 1991), but the chloroplast structure and division are quite different. In its chloroplast morphology and division, N. ulvacea corresponds closely to members of the Cymbellales D. G. Mann (in Round et al, 1990).…”

“…Triploid auxospores have also been reported in Gomphonema parvulum var. micropus (Kfitzing) Cleve (Geitler, 1932), Cocconeis placentula (Geitler, 1927b) and Craticula cuspidata (Kftzing) D. G. Mann (Mann & Stickle, 1991), and occur too in Amphora libyca Ehrenberg (sensu Krammer & Lange-Bertalot, 1986: my unpublished observations). In the last-mentioned, putative tetraploids have been seen with initial valves.…”

“…51(F)e,f), and this presumably protects the gametes and young zygotes while they are still naked protoplasts. Swelling seems to be the main mechanism in Dickieia for bringing the gametes into contact, as it is also in Craticula (Subrahmanyan, 1947;Cohn et al, 1989;Mann & Stickle, 1991). Elsewhere, bodily movement is more important, either of individual gametes, as in some Nitzschia Hassall species (Geitler, 1928;Mann, 1986), Gomphonema (Geitler, 1932), or Caloneis (Mann, 1989a), or of coordinated pairs, as in Neidium (Mann, 1984b) or Cymbella cornuta (Ehrenberg) Ross (unpublished observations).…”

“…In many other diatoms there is no plasmolysis and the perizonium directly moulds the shape of the initial cell (e.g. Neidium, Caloneis, Sellaphora, Craticula: Mann, I984b, I989a, b, 1994b;Mann & Stickle, 1991). In general, it seems to be rarer for freshwater diatoms to modify their shape during formation of the initial cell than for marine ones, just as it is rare for freshwater diatoms to divide non-interactively during mitotic division (i.e.…”

“…It is still rare for taxonomic papers on diatoms to concentrate more on the protoplast and reproductive behaviour than on dead cell walls. The latest revision of the recent Siisswasserflora yon Mitteleuropa (Lange-Bertalot, 1993), for example, is still wholly orientated towards frustule morphology, although it does accept a genus (Craticula Grunow) that was re-established largely because of differences from Navicula Bory in auxosporulation and protoplast structure (Mann & Stickle, 1991). I hope that the present paper will be seen as further confirmation of the value of'non-standard' characters in diatom systematics (see also Hasle et al, 1983;Cox, 1987Cox, , 1993Mann, 1990Mann, , 1993 and will encourage others to make the non-siliceous parts of the cell a priority for study.…”