Nr. 1] SUBAERIAL ALGAE FROM SOUTH AFRICA 11) 
forma neque magnitudine a cellulis vegetativis diversis ortis. Per 
porum membranae liberantur. 
This very characteristic and readily distinguished species OCCUTS 
as a thick, greenish inerustation on the bark of trees in several of 
the samples collected. It ocecurs very frequently associated with 
Myrmecia globosa, and also often with Trentepoklia lagenifera var. 
africana. 
One of the most conspicuous characters by which this alga 
differs from the already known species of the genus — as they are 
drawn and described by Cropat in Mat. Hist. Prot. I (1894), and 
Snow, Pseudo-Pleurococeus(1899)—isthatthethallusin Pleurastrum 
constipatum nov. spec. is generally larger and more compact, con- 
sisting of several layers of thick parenchymatous cell-masses, with 
a more even and smooth outline. The shape of the thallus, further- 
more, is very varying, from almost spherical complexes destitute of 
fiilaments or nearly so, through all transitions to specimens in which 
the thallus has produced plenty of filaments. On pl. IV is drawn a 
series 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- 
strucetion, 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 younger and newly 
formed cell walls are thinner, but grow with age gradually in 
thickness. (Vide e. g. pl. III, fig. 134). In specimens in vivacious 
division the cell-walls therefore are comparatively thin, in other 
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 u. Young cells shortly after the division may measure 
down to 3,5 pu, Just as older cells, on the other hand, may reach a size 
of 9 u in diameter, which seems to be the limits. On treatment with 
Flor iodide of zinc the cell-walls get a bright, reddish-violet tint. 
Like the other species of Pleurastrum, this one also emits irre- 
gular filaments, which here are very short and thick, with almosi 
isodiametrical cells. The cells in the filaments are never lengthened. 
The chromatophore is bell-formed, 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 being of 
any particularly decisive systenratic value, but I regard this feature, 
as being a distinet eriterion 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 cells. 
