Freshwater algae of Southern Africa . IV . Some Micrasteriae from Rhodesia , including a new species

RESUM E ALGUES DULCICOLES D'AFRIQUE AUSTRALE. IV. QUELQUES M ICRASTERIAE DE RHODESIE, IN C L U A N T UNE N OUVELLE ESPECE On discute Micrasterias ambadiensis (Gronblad & Sco tt) Thomasson, M. crux-melitensis (Ehrenberg) Hassall forma minor Turner, M. decemdentata (Nageli) Archer, M. pinnatifida (Kutzing) Ralfs var. incudiformis West & West, M. radiata Hassall var. brasiliensis Gronblad sensu lato, et M. tropica Nordstedt var. tropica. Une nouvelle espece est decrite: M. schweickerdtii Claassen.


METHODS
The material was preserved in 4% formalin.Slides were made by mounting a sample droplet in a drop of glycerine.
Drawings were made with a Zeiss binocular bright field/phase contrast microscope using a Leitz micrometer-net-ocular and specially printed squared paper, the squares being 2x2 cm.The lenses used were a 12,5x eye-piece and a 40x objective.
Photomicrographs were taken on Adox KB14 film using a 35 mm Willd microscope camera on a Zeiss Nomarski contrast microscope.A 6x eye-piece and a 40 x objective were used.
All dimensions are given in micrometres (//m).

OBSERVATIONS A N D DISCUSSIO N
The following taxa were identified in the sample collected by Schweickerdt: 1.
Micrasterias ambadiensis (Gronblad & Scott) Thomasson (1960, p. 22, Figs 4: 10 & 10: 4;Lind, 1971, p. 542, PI. 2, Fig. 3).Some variation with regard to the delineation of the lateral lobes occurred in the Rusape specimens .In the type figures by Gronblad & Scott (1958, Figs 119 and 363) as well as Thomasson's figure (1960, Fig. 4: 10) the inferior lateral lobes have three spines each and the superior lateral lobes either three or four.In the specimen depicted in Gronblad's Fig. 9 (1962)  in Lind's specimen (1971, PI. 2, Fig. 3) both inferior and superior lateral lobes have three spines each.In the Rusape specimens the number of spines in both inferior and superior lateral lobes varies from two to four.In a single specimen one inferior lateral lobe was completely absent and another consisted of a single spine (Fig. 12, bottom semicell).An abnormal coalescence of the cell wall in the inner portion of the sinus was observed in another specimen (Fig. 26).This specimen also showed abnormal cell wall thickenings at the base of each incision between the inferior and superior lateral lobes and between the superior lateral lobes and the polar lobe (upper semicell) as well as a large granule in the centre of the apical margin of the polar lobe.Length without spines 114-136, with spines 140-168; width without spines 98-113, with spines 112-131; width of isthmus 23-24; maximum width of polar lobe without spines 44^8, with spines 60-72.Very common.
5. Micrasterias radiata Hassall var.brasiliensis Gronblad sensu lato, 1945, p. 15, PI. 4, Figs 82 & 83;Forster, 1969, p. 41, PI. 11, Fig. 5 & PI. 12, Fig. 1;Thomasson, 1960, p. 24 The plant referred to here and all the cited figures seen by the present author more closely resemble Gronblad's Fig. 83 than his Fig. 82.A broad inter pretation of M. radiata var.brasiliensis has been adopted because, as the following discussion reveals, there is no name available for the plant illustrated in Fig. 83, and it is felt that, in view of the rather confused nomenclature, it would be unwise to describe a new taxon at this stage.
When Gronblad (1945) created M. radiata var.brasiliensis he illustrated it with two figures.In his Explanation of Plates he said "Fig.82 is the most frequent form, Fig. 83 is a larger form with more slender processes" (Scott, Gronblad & Croasdale, 1965, p. 40).Croasdale (Scott, Gronblad & Croasdale, 1965) felt that the plant represented in Gronblad's Fig. 83 should be distinguished from the plant represented in Fig. 82.She included this plant (Fig. 83) in the new variety M. radiata var.groenbladii Croasdale (Scott, Gronblad & Croasdale, 1965, p. 39, PI. 5, Fig. 80) and thereby it could possibly be con strued that she indirectly made Gronblad's Fig. 82 the lectotype of M. radiata var.brasiliensis.As Croasdale failed to indicate a type, the name M. radiata var.groenbladii was not validly published (Stafleu, 1972, p. 41, Art. 37). Forster (1969, p. 41) decided that the plants depicted in Gronblad's Fig.  Forster (1969, p. 41, PI. 12, Fig. 2) as a synonym.The name M. radiata var.croasdaleae was also not validly published as no type was indicated, although Forster mentioned on the first page of his paper where slides containing new described taxa are preserved.
In the Rusape specimens there is a noticeable variation in the width of the basal portions of the polar lobes of mature semicells (Figs 9,10,24 and 25).An abnormal coalescence of the cell wall was observed in the inner portion of the sinus (Fig. 24) between the superior lateral and polar lobes (Fig. 24) and between the lobules of the inferior and superior lateral lobes (Fig. 25, mature semicell).The latter also showed abnormal cell wall thickenings in the diverging processes of the polar lobe.Length with spines 180-208; width with spines 138-150; width of isthmus 20-22; maximum width of polar lobe 80-100.Common.
The specimen depicted in Figs 3, 4 and 8 was slightly anomalous in that one side of the sinus was linear and not open. 7.
In the polar lobes of the Rusape specimens (Figs 22,23,30 and 31) the diverging processes are longer than in the type.These specimens fall rather in between the plants represented in Krieger's Fie. 4 (1939, PI. 112) andhis Fig. 7 (1939, PI. 113) for M. tropica var.elegans West & West as the lateral and polar lobes are more slender than in the typical variety and less slender than in var.elegans.The semicell depicted in Fig. 23 could be compared with var.elegans but for the shorter polar lobe.Length 92-104; width 80-106; width of isthmus 14-17,7; maximum width of polar lobe 58-63,5.Rare." 'S .
both inferior and superior lateral lobes have four spines each and * Department of Botany, University of Pretoria, Pretoria.
and it is not clear whether he meant that his plant was M. radiata var.Forster is superfluous and illegitimate and Forster's concept of M. radiata var.groenbladii brasiliensis Gron blad, pro parte, quoad Fig. 83.Later Croasdale (Gronblad & Croasdale, 1971, p. 40) indicated her Fig.80 in Scott, Gronblad & Croasdale, 1965, as the type for M. radiata var.groenbladii Croasdale.This validated the publication of the variety and means that the name M. radiata var.croasdaleae