1897 | SOME NEW SPECIES OF MINNESOTA ALG 97 
a somewhat different structure (g/. V///). The incrustation 
just described now appears as the substratum, its surface being 
covered by the thalli of Chetophora calcarea, which was distrib- 
uted as no. 11 in American Alge, Century 1. 
The thalli project from the substratum. They form somewhat 
globose mounds, or later these are confluent into sharp ridges or 
shelves parallel to the surface of the water. These shelves may be 
compared in shape to a Polyporus and are peculiar in construc- 
tion. The upper portion consists of the Chetophora thalli 
proper, being in color a chlorophyll green; the substructure is 
made up of the blue-green species, notably the larger Lyngbya, 
which causes the bright purplish-blue color. Evidently, in the 
beginning, the Chztophora thallus is solitary, has a globose 
form, and stands out at right angles to the substratum, thus 
presenting one side to the direct light of the sun, while the 
opposite side is in the shade. The Lyngbya seizing the oppor- 
tunity offered for additional room and indirect light soon forms a 
growth upon the under side of the Chetophora thallus. This in 
turn takes advantage of the support given by the Lyngbya, 
which it uses as a substratum, and takes an upright position to 
receive on all sides alike the direct sunlight. Thus the two 
plants develop, keeping pace with and aiding each other, until 
eventually the above mentioned structure is formed. It may be 
said that the Lyngbya forms a shelf upon which the Chetophora 
thallus may rest, or that the Chztophora makes of itself a 
screen for the protection of the Lyngbya. This is a distinct and 
somewhat peculiar form of symbiosis. 
For a time it remained a problem why the Cheztophora 
should be confined to the three sides of the tank, while the 
blue-green plants occupied also the fourth side. With some 
difficulty the position of the inlet of the tank was located. It 
was found that the water enters in the corner facing the south, 
that it flows out again at the west corner in a stream a foot in 
width, almost immediately disappearing in the ground. From 
this it appears that there is a current along the southwest side 
of the tank. Elsewhere the water, while not eau is not 
