78 SMITH: TETRADESMUS, A NEW COENOBIC ALGA 
in weaker nutritive solutions the chromatophore of Scenedesmus 
acutus does not fill the entire cell but that there is a hyaline region 
at one side. My experiments with Scenedesmus acutus confirm 
this point; however, the chromatophore in Tetradesmus always 
completely fills the cell. The pyrenoid of Scenedesmus is con- 
stantly somewhat larger than that of Tetradesmus, being about 2.5 
uw in diameter in the former and 1.5 w in the latter. The arrange- 
ment of the starch grains differs somewhat, the characteristic 
segmentation of the grains around the pyrenoid being easily seen 
in Scenedesmus and only with great difficulty in Tetradesmus. In 
the older cultures of the former species the cell is usually filled 
with characteristic angular pieces of ‘‘stroma’”’ starch that have 
been derived from the pyrenoid; in Tetradesmus kept under similar 
conditions the ‘“‘stroma”’ starch granules are not very abundant. 
A good idea of the cellular structure of Scenedesmus acutus may be 
obtained from FIG. 3. 
Among other coenobic algae the forms which appear to re- 
semble Tetradesmus most closely are Selenastrum Reinsch (17) 
and Lauterborniella Schmidle (13). Selenastrum, as described by 
West (17), differs from Tetradesmus in that there are more than 
four cells in some of the colonies, and that the individual cells do 
not possess a pyrenoid. Dr. Schmidle has kindly compared my 
material with his type specimens of Lauterborniella and informs 
me that neither in size, shape, nor arrangement of the cells does 
Tetradesmus resemble Lauterborniella. 
MorPHOLOGY 
The individual cells of the Tetradesmus colony are always more 
or less ovoid, but in every case they are sharp-pointed at the ends. 
The cell wall is composed of two parts, as Chodat and Malinesco 
(4) have shown for Scenedesmus. The inner part is of cellulose 
and reacts to zinc chloriodid; the outer is composed of a gelatinous 
material which serves to bind the cells of the colony together. 
The existence of this outer gelatinous layer can be demonstrated 
by the use of some such stain as Bismarck brown. There is no 
central vacuole, but the entire cell except for the nucleus and the 
pyrenoid is filled with a cytoplasm which is granular or rarely 
slightly alveolar in structure. The chlorophyl is distributed 
