PLANKTON ALG OF LAKE ERIE. 383 
As the type species of Plewrococcus, Plewrococcus vulgaris Menegh., reproduces by means of 
simple vegetative division involving both contents and membrane, it would seem that this species 
could not rightly be called Plewrococcus. In respect to the mode of reproduction it agrees with 
Chlorella, but the presence of the thick, gelatinous envelope is not characteristic for that genus. 
The physiological characteristics also of this species vary widely from those of Chlorella. The 
correct systematic position seems to be near to AKirchneriella, as the chief point of distinction 
between the two forms is the shape of the cells, the cells of this species being spherical, while the 
cells of Kirchneriella are crescent-shaped. The formation of cell complexes is the same. 
Pieurococcus aquaticus Snow. new species. 
Pleurococcus aquaticus shows in its highest development the typical structure of Plewrococcus, 
consisting of clusters of cells 2, 4, 8, 16, 32, or even more in number, arranged in cubical form 
(fig. x, 1). These clusters arise from the repeated division of cells, alternating in three directions 
of space. 
The diameter of the cells of a cluster varies from 4 to 7 4. The membrane is thin and gives 
no reaction with iodine and sulphuric acid. The chloroplast is single, concave, thin, parietal, a 
vivid green in color, and has an opening on one side, which, however, is rarely distinguishable 
while the cells are arranged in the cluster. No pyrenoid is present. The small spherical or oblong 
nucleus lies near the center of the cell. 
The large clusters of cells evidently do not increase indefinitely in size, for after a period, 
under ordinary conditions, the cells undergo a dissociation, the contact between them becomes 
destroyed, and the cells fall into formless masses (fig. x, 5). The individual cells may then divide 
and either produce again the large clusters (fig. x, 1, 4), or they may divide and remain in the 
isolated state in which they were (fig. x,2). It has been noticed that if the cells are some distance 
apart they produce again the large clusters, but if they are closely crowded, instead of remaining 
-united after division, they become separated and exist as many single cells, which, except for the 
absence of a pyrenoid, can with difficulty be distinguished from an ordinary Chlorella vulgaris 
Beyerinck. They are spherical or, before division, ellipsoidal, and the opening on one side of the 
chromatophore is conspicuous. At any time these cells, when separated from each other, again 
form large clusters. In large cultures both single cells and clusters were usually present, and in 
only one instance were the single cells wholly lacking. This was ina culture started for other 
purposes in a tube made from collodion, similar in shape toa test tube. This was. filled with a 
0.2 per cent Knop’s solution, the algee inserted and the tube sealed. The whole was then immersed 
in a 0.2 per cent Knop’s solution. After a few weeks the increase had been great, but only the 
large masses were present. Why the alga did not undergo a dissociation in this mode of 
cultivation, as well as in others, was not determined. The cells were also somewhat larger than in 
ordinary cultures, all having a diameter of 6.5 to 7 ye (fig. x, 3). 
This species was in no case found in fresh material, but was found in a large plankton culture 
and also in a culture taken from washings of Chara growing among Scirpus americana Pursh. and 
Sagittaria rigida Pursh.,in Squaw Bay, South Bass Island. As it was found in but a single 
plankton culture, it is probable that it is one of the many littoral forms which at times are found 
in the plankton, and that it had been carried out into the plankton by the action of the water. As 
it was never seen except in culture, it is difficult to say in what condition it exists m the natural 
state. whether as isolated cells or in large cell complexes. It is probable that the dissociated form 
is more usual, as the large cell complexes would be less apt to be overlooked. 
Chlorococcum natans Snow, new species. 
The cells are spherical or slightly ellipsoidal, the greatest diameter noticed being 13 u.. In 
appearance they resemble somewhat Chlorococcum infusionum Menegh., but are smaller in size, 
are of a lighter, more transparent green, and the contents, instead of being granular, are mottled, 
due to thicker and thinner places in the chloroplast (fig. x1, 1). 
The shape of the chloroplast is a hollow sphere through which is a circular opening. On the 
side opposite this lies a pyrenoid with a starch envelope. The membrane is of cellulose. In the 
young stages there is a single nucleus, but shortly before reproduction the nucleus divides so that, 
for a period, the cells are multinucleate. 
