A Igae and the Principles of their Evolution . 1 1 5 
simplex , Artari 1 ), as well as such very different forms as Cysto - 
coccus and Protococcus vulgaris. 
This plant having been considered as the type of the so- 
named ‘ Pleurococcaceae’ by recent authors, such as Klebs,Wille, 
&c., it is necessary to give somewhat full details concerning it. 
As generally described in its natural and commonest con- 
dition, the four cells are arranged in one plane, and each cell 
is characterized by a parietal chromatophore. By repeated 
divisions very regular thalli are formed, in which the cells lie 
close together. This very common condition of Pleurococcus 
vulgaris is the aerophilous form, which produces a more or 
less thick stratum on the bark of trees. In a nutritive solution 
the culture of this plant, gathered in very dry places, is often 
impossible. When, however, in such a solution, the aeration 
is good, some, or all, of the individuals grow very well, and 
after some days or weeks the formation of a short filament 
begins by the protrusion of a prolongation continuous with 
the Pleurococcus-czW. In some cases, out of the four or eight 
cells of a thallus, only a single one produces such an ex- 
pansion, so that it is unquestionable that the filament belongs 
to a true Pleurococcus vulgaris. In other instances from two 
to eight do the same. The growth of the filaments is always 
slow, and when they become more elongated division by 
segmentation occurs. This may also be observed in larger 
thalli which are covered with short or long hairs arising from 
nearly all the outer cells. When circumstances are favourable 
almost all the individuals develop in the same way, and a 
great many little branched or unbranched filaments spread 
in every direction. I have not been able to obtain filaments 
exceeding twelve cells in length ; the thalli often appear to 
be more branched, but this is due merely to the fact that the 
four initial cells of the normal Pleurococcus from which the 
filaments originated still remain united ; in this case the rami- 
fication is radiating. I have repeated these cultures a great 
number of times, and have always obtained the same result. 
1 Loc. cit., p. 27. 
I 2 
