PLANKTON ALG^E OF LAKE ERIE. 
877 
ture soon becomes more or less broken and distorted, as is seen in the material from the plankton, 
and the irregularity increases as reproduction continues. As the gelatinous substance which 
holds these together is invisible without reagents, one receives the impression that each plate-like 
mass is a ccenobium or individual, whereas in reality it is made up of many. 
Under normal conditions of growth the cells of this species are ovoid or lemon-shaped, with the 
membrane projecting into a very short and almost obscure wart at one or both ends of the cell 
(fig. v, 4, 5). The typical shape is evidently that of a lemon which is more or less unsymmetrical 
with reference to its long axis. But in the ccenobia where two cells are in contact their opposed 
ends often become more or less flattened. The projecting portions of the membrane are then often 
not formed, and the cells are quite distinctly ovoid, the broader portions of the adjoining cells 
turned toward each other. The mature cells measure 5.3 to 8 a long and 3.25 to 5.3 a broad, while 
in young ccenobia they are 5 to 5.8 a long and 3.25 to 4 / 1 broad. 
The membrane is very thin and consists of cellulose as shown when treated with iodine and 
sulphuric acid. Surrounding the membrane and enveloping the whole ccenobium or compound 
coenobium is the homogeneous gelatinous substance which unites the separate ccenobia. This is 
apparently excreted from the cells and is not a dissolved portion of the membrane, as the mem- 
branes of one or more preceding generations are sharply defined and lie embedded in this substance 
(fig. v, 1). The ways in which this responds to different stains are various. Fuchsin-iodine 
green neither stains the gelatinous substance nor is capable of penetrating it. It therefore leaves 
the cell contents uncolored. Haematoxylin, fuclisin, and safranin stain the cell contents, but not 
the gelatinous envelope; the latter, however, takes a deep color with gentian violet. The structure 
is best brought out by tannate vesuvine which stains it brown. With this stain single ccenobia 
show but a single layer of this substance, often thicker than the diameter of the cells themselves, 
though varying somewhat in amount. In the large compound coenobia several layers are made 
visible, each successive outer layer being less dense than the adjoining inner layer. These different 
layers are the gelatinous envelopes developed during the different generations and retained from 
one generation to the next. The inner denser layer is sharply outlined from the others and is 3 to 
3.5 // thick. With the tannate vesuvine fine radiating lines are brought to view at right angles to 
the surface of the cell, and undoubtedly indicate a prismatic structure (fig. v, 3) such as described 
by Klebs ’86 for Zygnema. The second or next outer layer shows no such striations, but is quite 
definitely outlined, while the third and outermost visible layer is more or less indistinct and 
gradually vanishes into the surrounding medium. 
The chloroplast is thin, parietal, and forms a close lining to the membrane. In some young 
coenobia, on the side of the cell next to the central space, there was seen to be an opening through 
the chromataphore, but in mature specimens no trace of an opening could be detected. Lying 
imbedded in the chloroplast is a single, relatively large pyrenoid surrounded by a thick layer of 
starch. The position of the pyrenoid in the cell is in no wise constant, as sometimes it lies nearer 
one end of the cell and sometimes nearer the other. Its position also in reference to the nucleus 
is not constant; the latter, however, occupies different positions in the cell according to the age. 
In young coenobia the nucleus invariably occupies a position near the wall adjoining the cen- 
tral space (fig. V, 6a) , while in older individuals it moves toward the center or near to one end of 
the cell. The minuteness of the nucleus renders a detailed study difficult, but material stained 
with hsematoxylin showed strands of protoplasm radiating from it, and in one case a nucleolus 
was plainly visible (fig. v, 8). In the same material also a stained network appeared throughout 
the cell and was undoubtedly due to the arrangement of the protoplasm and vacuoles. Small 
globules of oil were always present, and occasionally larger globules. This oil became darkened 
by osmic acid and was dissolved in 10 per cent potassium hydrate. It was not dissolved in absolute 
alcohol, which would show the oil to be of a fatty rather than an ethereal nature. 
The new individual arises from the successive bipartition of the contents of any cell of the 
coenobium. The first division is a transverse one (fig. v, 4). The second division, which occurs 
in each of the products of the first division, is at right angles to the first and in the same plane in 
the two products, so that the elements are arranged in the form of a rectangle while still within 
the mother membrane. The division of the different contents of the cell is not simultaneous, as 
the chromataphore is divided before the pyrenoid, and judging from their relative position in the 
cell at this time, both of these divide before the nucleus. The four daughter nuclei occupy a 
position near the point of contact of the four cliromataphores (fig. v. 4a) , while the pyrenoids lie 
