PLANKTON ALGA3 OF LAKE ERIE. 
387 
Mesocarpus sp. 
This small species of Mesocarpus was found so often in the plankton that it might almost be 
called a plankton species. At first it could not be recognized as Mesocarpus, for usually the 
chlorophyl was greatly reduced and was collected in a very small space at the center of each cell 
(fig. xvi, 2) , and it was only after the form was placed in culture that it was recognized as belong- 
ing to that genus (fig. xvi, 1). As the zygospores were never found, the species could not be 
determined with certainty. By means of cultures « in a large number of media it was determined 
that the chlorophyl collects under any circumstances that are not favorable, such as in too weak or 
or too strong culture media (1 per cent or 0.05 per cent Knop’s solution), or in media that are not 
adapted to the plant, as well as in old cultures when the nutrition has become exhausted. It 
would appear, then, that when this form occurred in the open lake, the nutrition was not quali- 
tatively or quantitatively favorable to its most vigorous condition, though adequate to maintain 
its existence and even growth. 
Coelospheerium roseum Snow, new species. 
Several forms of Ccelosphcerium are almost universally found in the plankton during the 
summer months. Some of them are easily recognized, while others are of doubtful name, and the 
difficulty of cultivation makes the classification more difficult. One of those most commonly 
found is the form shown in fig. xvii, 1. The colonies are 34 to 52 /i in diameter; the cells measure 
3.25 to 4 /< in diameter, are spherical, pinkish or brownish in color, and are closely arranged or some- 
what scattered over the surface of the gelatinous sphere. Indigo solution shows the presence of a 
gelatinous covering to the colony, varying in thickness according to conditions. On agar cultures 
this covering assumed a diameter almost equal to that of the colony. 
If the cells are closely arranged, and if the focus of the microscope is on the surface of the 
sphere, the colony appears as a typical Ccelosphcerium where the gelatinous sphere is homogeneous 
(fig. xvii, 1 a) , but if the cells are less closely arranged and the focus is at center the gelatinous 
portion may be seen not to be homogeneous, but to consist of a system of dichotomous gelatinous 
branches radiating from a common center, bearing the cells on their terminal divisions (fig. xvii, 
1 b ) , as in Dictyosphcerium. Between these branches lies gelatinous substance continuous with 
that surrounding the colony. The line of demarkation between this substance and the branches is 
more or less distinct, according to conditions. In some cases it is hardly distinguishable, while 
in others it is sharply defined. 
As a solution from a quantity of Ahabcena flos-aquce was the only one in which vigorous 
cultures could be obtained, the alga could not be subjected to a very great variety of conditions, 
and yet simply transferring from the lake water to this solution produced some change. Always 
in cultures the cells became more closely arranged, the branches became indistinct, and in many 
cases, apparently when the conditions were less favorable than in the lake, the color changed from 
a pink to a brown, though if the concentration of the solution were right the pink color was 
maintained. Apparently the distinctness of the branches is controlled somewhat by the density 
of the surrounding medium — the denser the medium the denser the surrounding substance, and, 
consequently, the less conspicuous the branches. Distilled water seems to be a solvent of this 
substance and the cells become detached entirely from the colony. The reproduction of this species 
is typical in all respects for Ccelosphcerium as described by Naegeli. If the cells become detached 
from the stalks, then division occurs once in three directions of space, and afterwards only in two 
directions, both at right angles to the surface of the sphere. As no very small colonies were found 
in the plankton, however, it is probable that in nature reproduction usually occurs by the division 
of the entire colony into two, rather than that new colonies originate from the separated cells. 
Both forms mentioned — the pink, with the visible gelatinous branches, and the brown, where 
the gelatinous branches are not visible — were at times found in the plankton, but their identity is 
evident. A number of others differing slightly from either of these were found which possibly 
may have been connected with these, but as neither could be made to assume the characteristics 
of the other artificially, their identity as yet can not be assumed. In one case all traces of the 
enveloping gelatinous substance were wanting, and the cells, borne on what seemed to be perfectly 
free gelatinous stalks, were moved about freely by the action of the water (fig. xvii, 2). Other 
u These cultures were continued for some weeks by Miss Anna L. Rhodes, as well as by the writer, 
