Nr. 1] SUBAÉRIAL ALGAE F ROM SOUTH AFRICA 19 



formå neque magnitudine a cellulis vegetativis diversis ortis. Per 

 porum membranae liberantur. 



This very chaiacteristic and readily distingiiished species occui's 

 as a thick, greenish incrustation on the bark of trees in several of 

 the sampiles collected. It occurs veiy frequently associated witli 

 Mijrmecia globosa, and also often wilh Trentepohlia lagenifera vai\ 

 africana. 



One of the most conspicuous characters by which this alga 

 differs from the ah-e-ad}' known species of the genus — as they are 

 drawn and described by Chodat in Mat. Hist. Prot. I (1894), and 

 Snow, Pseudo-Pleiirococciis i;i899) - is that the thallus in Pleuraslrum 

 constipcdum nov. spec. is generalh^ larger and more compaot, con- 

 sisting of several layers of thick parenchymatous cell-masses, witli 

 a more even and smooth outline. The shape of the thallus, further- 

 more, is very varymg, from almost spherical complexes destitute of 

 filaments or nearly so, through all transitions to specimcns in which 

 the thallus has produced plenty of filaments. On pi. IV is drawn a 

 sei'ies of thalli which will give an idea of their appearance and 

 structure. The size also varies considerably, from quite small, few- 

 celled up to very large ones, where the thallus is formed by several 

 hundreds of cells, polyedrical and nearly isodiametrical, lying in 

 no real order. Some few specimens show a more regular con- 

 struction, however, on account of more regular cell-divisions. The 

 cell-walls are comparatively thick, especially so the older ones, and 

 those on the surface bounding the thallus. The yomiger and newly 

 formed cell walls are thinner, but grow with age gradually m 

 thickness. (Vide e. g. pi. III, fig. 1,34). In specimens in vivacious 

 division the cell-walls therefore are comparatively thin, in olher 

 specimens that are not in such a state of rapid division, they are 

 comparatively thicker. The size of the cells is very varying, on an 

 average 6 — 8 «. Young cells shortly after the division niay measure 

 down to 3,5 /<, just as older cells, on the other hånd, m/ay reach a size 

 of 9 jii in diameter, which seems to be the limits. On treatment with 

 chlor-iodide of zinc the cell-walls get a bright, reddish-violet tint. 



Like the other species of Pleurastriim, this one also emits irre- 

 gular filaments, which here are very short and thick, with almost 

 isodiametrical cells. The cells in the filaments are never lengthened. 

 The chromatophore is bell-fomied, parietal, and lines all the 

 membrane without leaving irregular openings anywhere. The 

 species, in addition, is characteristic in being without a pyrenoid. 

 The presence or want of a pyrenoid I do not consider as bemg of 

 any particularly decisive systematic value, but I regard this feature, 

 as being a distinct criterion between species. In each cell is one 

 central nucleus. 



The reproduction is done by zoospores, formed in zoosporangies. 

 which, as to shape and size, do not differ from the vegetative oells. 



