Valve Width Research Articles

SEXUAL REPRODUCTION AND AUXOSPORE FORMATION OF THE MARINE MONORAPHID DIATOM COCCONEIS PELLUCIDA

Sexual reproduction and auxospore formation of Cocconeis pellucida have been observed using a laboratory culture. This diatom was monoecious. Auxospores occurred at 18°C and 14°C, but not at 5°C. One gamete is formed in a gametangium, without cytokinesis. Each gametangium secretes gelatinous material near where it contacts the other gametangium. Sexual reproduction occurs via behavioural anisogamy and the single auxospore expands spherically. Sexual reproduction and auxospore formation corresponds to Type IIB1 of Geitler's system. The perizonium consists of transverse bands, which exhibit a marked dorsal angle on the dorsal side of the auxospore. The open ends of the transverse bands converge towards the centre of the ventral side of the auxospore, instead of forming a suture as in other pennate diatoms. The auxospore forms first a raphid concave valve and then an araphid convex valve. The initial cell is like a vegetative cell, except for the presence of two plastids. C. pellucida differs from C. placentula in the presence of a raphid valve in the initial cell, the absence of unequal cytokinesis during gametogenesis, delay in the division of the initial cell until after it has left the perizonium, and survival of both daughter cells following division of the initial cell. Cocconeis is unique among pennate diatoms in the formation of a spherical auxospore and die proportional decrease of valve length and width during the life cycle, which are otherwise characteristic of centric diatoms.

MORPHOLOGICAL VARIATION OF THE ATTACHED DIATOM:COCCONEIS SCUTELLUMVAR.SCUTELLUM: (BACILLARIOPHYCEAE)1

ABSTRACTThe morphology and sexual reproduction of the attached diatom Cocconeis scutellum Ehr. var. scutellum have been investigated using field and culture materials. The diatom grew well at 14–22° C and in a medium using enriched full strength seawater or 50% diluted seawater. One gamete was formed without cytokinesis in a gametangium and one auxospore arose from paired gametangia. The size of gametangia and initial cells were 17.8–29.9 μm and 43.3–59.5 μm, respectively. Valve width decreased linearly with decreasing valve length. The regression coefficient between them was invariable with the change of the environment. The transapical striation densities were 7–12 and 7.5–13.5 in 10 μm in araphid and raphid valves, respectively. The density showed a tendency to increase with decreasing valve length in both valves. The densities in both valves, except the small araphid valve, are invariable with the change of the environment. The relationships between valve length and width, and between valve length and striation density have been compared with those of taxa allied to C. scutellum var. scutellum. It is concluded that C. scutellum var. scutellum, as usually delimited, is a heterogenous taxon.

Palæontological Notes on the Silurian Inlier of Woolhope

Shell ovate, with sides subparallel and very slightly arched; anterior end broadly rounded, subtruncate. Pedicle-valve with umbonal edges diverging at about 110°. Interior of pedicle-valve with a large, elongated, subrhomboidal, thickened area extending for nearly half of the length of the valve and produced in front into a long, tapering point reaching to about three-fourths of the length of the valve; area defined by narrow raised ridges which arise near the beak, and run outwards and forwards on each side with increasing strength and slight curvature to the widest part of the area, thence curving inwards and converging, at first at about 60° and finally at a rapidly diminishing angle, to meet in a sharp point in the middle line. Concrete laterals narrow, triangular, extending for more than half of the length of the area, forming an elongated, subrhomboidal scar divided by a rather broad, rounded median groove continued forwards to the anterior point of the area; centrals transverse, oval, oblique, large, lying in front of and alongside of the concrete laterals, separated anteriorly by the median groove. Cardinal margin of valve somewhat thickened and flattened internally. Dimensions.—Length of valve = 26 mm. Width of valve = 19 mm. Length of area to tip=17 mm. Width (maximum) of area = 9 mm. Horizon and locality.—Woolhope Limestone, Littlehope. Remarks.—The internal characters of this shell are closely similar to those of L. lewisi Sowerby, from the Aymestry Limestone; but the internal area is more developed as a platform, the median ridge is