The Triradiate and other Forms ofNitzschia Closterium(Ehrenberg) Wm. Smith, FormaMinutissimaof Allen and Nelson

Abstract

1. The natural history in culture of the species of Nitzschia originally isolated from the sea by Allen & Nelson (1910) and grown at Plymouth and elsewhere ever since has been studied. It is shown that three main types of cell exist: normal with two arms (rostra), triradiate with three arms and ovals with none. Both normals and triradiates produce ovals by division, and the ovals so produced can multiply to form further ovals, or can grow either two or three arms, generally two, to form normal or triradiate cells.2. Ovals are more readily produced under good lighting conditions than under weak illumination.3. The cell wall is very lightly silicified, and the cells, unlike those of most diatoms, can elongate without auxospore formation. The maximum, and with it apparently the average, cell size of successive subcultures is variable.4. When triradiates are present they may, and frequently do under good cultural conditions, multiply faster than normal cells and finally may almost completely dominate the cultures. Now and again a proportion of the triradiate cells shorten one arm and gradually pass, during successive divisions, into the normal condition. There is a greater tendency for this process to take place in the autumn than at any other time. There was a notable instance in November 1941 when several cultures at Plymouth and at Hull simultaneously produced normals by this method on an extensive scale.5. There is evidence that under unfavourable cultural conditions triradiates tend to be eliminated.6. A cruciform type of cell has appeared occasionally in several cultures, but it has not been possible to obtain pure cultures of it, for the type appears to be somewhat unstable and readily reverts to the other forms. It has, however, persisted for a long time and for very many generations.7. It is shown that the triradiate type occurs in nature, and there is no reason to suppose that it is an abnormal form due to culturing.8. The nomenclature is discussed and it has been decided for the time being to retain the name Allen and Nelson originally gave to it, although there is a possibility that it is a species separate from Nitzschia closterium (Ehr.) W. Smith.

Observations on Phaeodactylum tricornutum

“…Ten days later all the cultures had produced abundant fusiform cells, Since no triradiate forms were seen, this form evidently does not persist beyond the intermediate stage of the oval cells. This confirms observations by Wilson (1946) and by Droop (Hendey, 1954), and supports the view that triradiates are not a normal stage of the life cycle. These studies are being continued with a view to determining what factors may be responsible for the changes in cell form and whether a sexual cycle may be involved.…”

“…Triradiate forms originate rarely in the laboratory, but under certain conditions, as in static cultures, such cells may be favoured by selection, perhaps because of a decreased rate of sinking, and may overgrow the other forms. In addition to triradiates, cruciform cells (Wilson, 1946), curved cells, and other abnormal forms have occasionally been observed. When such a morphological abnormality arises, it reproduces clonally, forming daughter cells of the same shape as the parent cell, since cell division takes place in a longitudinal plane.…”

“…I n 1942 Wilson & Lucas reported that triradiate forms of this organism arose independently in cultures maintained in Plymouth and in Hull. I n a detailed illustrated account of this unusual organism, Wilson (1946) reported the presence of fusiform, triradiate and cruciform cells, and observed that both fusiform and triradiate cells sometimes gave rise to ovals. The ovals so formed either divided to produce more ovals or grew two or three arms to form fusiform or triradiate cells, respectively.…”

Filtering, feeding, and digestion in the lamellibranch Lasaea rubra

“…These results show a rapid and complete digestion of Isochrysis and Phaeodactylum. Whatever the true structure of the wall of Phaeodactylum (see Wilson, 1946, andHendey, 1954) it is evidently very easily attacked by digestion to release the protoplasmic contents of the cell. The majority of ingested Phaeodactylum had become' ghosts' in 45 min (Table XIV).…”

Note on the Plymouth ‘Nitzschia’ Culture

“…In 1939 the care of the cultures passed to Dr Douglas P. Wilson who published the first detailed account of the organism together with illustrations (Wilson, 1946).Wilson found the organism to be pleomorphic and described ovoid, fusiform, triradiate, and cruciform forms. Mter discussingthese forms in great detail Wilson concluded' Both normals [fusiform] and triradiates produce ovals by division, and the ovals so produced can multiply to form further ovals, or can grow either two or three arms, generally two, to form normal or triradiate cells ' (p. 268), and 'there is no doubt that as a general rule triradiates arise from normals onlythrough the intermediary of oval cells' (p. 251).…”