88 



THE JOURNAL OF BOTANY 



NiTzscHiA scALARis (Ehrenb.) W. Sm. This large species, 

 with its conspicuous markings, was frequent in the collection from 

 the Big Mosman Eiver. 



VIII. — Selenastrum acuminatum Lagerheim. 

 Selenastrum acuminatum Lagerh. in Wittr. & Nordst. Alg. 

 Exsic. 1882, no. 441 ; in Ofvers. af K. Vet.-Akad. Forh. 1882, 

 no. 2, p. 71, t. 3, f. 27-30. 



In his work on the Algues Vertes de la S^iisse, 1902, p. 211, 

 Chodat has placed Selenastrum acuminatum in the genus Scene- 

 desmus, an error which is apparently the result of having confused 

 Lagerheim's species with Scenedesmus obliqims var. dimorphus 

 (Turp.) Eabenh. The figures given by Chodat (I. c. p. 166, f. 88) 

 certainly represent the latter, whereas they do not represent 

 Selenastrum acuminatum. Unfortunately, those who have too 

 implicitly followed this work of Chodat have repeated the same 

 error. The latest instance of this is by Petersen, '^'- whose figure 

 of " Scenedesmiis acuminatus (Lagerh.) Chodat" is typical Scene- 

 desmus obliquus var. dimorpMcs. 



The colonies of Selenastrum acuminatum consist of only four 

 cells which easily dissociate. Each cell is lunate or arcuate, and 

 the acute extremities are suddenly drawn 

 out into long points, a character which at 

 once distinguishes this Alga from any form 

 of Scenedesmus obliqims (=Sc. acutus). In 

 the fully grown cell there is a single chloro- 

 plast with a slight excavation at one side, 

 and without a pyrenoid (fig. be). Occasionally 

 the chloroplast divides into two (fig. 5^). 

 Four autospores are formed in each mother- 

 cell by oblique longitudinal divisions. The 

 daughter-cells remain loosely attached for 

 some time, generally in rather an irregular 

 manner, but separate as the cells become 

 adult. 



The Alga was brought to me by Mr. 

 W. B. Grove from a small pond at Olton, 

 Warwickshire, where it occurred in immense 

 Selenastrum quantity from February to May, 1911. It 

 acuminatum Lagerh. a A, • -i • i i . Si. x xn i, 



and h, typical colonies of thrived m laboratory cultures from February 

 four cells; c-g, solitary to July, the best cultures being those in 

 cells. filtered, sterilized water from the pond in 



which it was originally found. 

 In a thriving culture the cells attained maturity in about 

 fifteen to twenty days, at the end of which period a good per- 

 centage of them produced a new generation of autospores. All 

 the colonies consisted of young cells, the mature cells being 

 invariably free. Thus this species is very closely akin to 

 Ankistrodesmus, being a connecting-link between that genus and 

 Selenastrum. 



* J. B. Petersen in Botan. Tidsskrift, Bd. xxxi. 1911, p. 171 (fig. 6). 



