148 
FIG. 2. 
the limit of life in these hot springs, and it 
is not likely to be found in water warmer 
than 85° C. 
Let us now consider the structure of 
some of the most striking types of plant 
life making up the vegetation of these hot 
springs. It would not be wise to go into 
details, and indeed I cannot well do so, 
for no one has ever published a critical 
study of these forms. The subject is really 
a very difficult one for several reasons. In 
the first place the organisms are all very 
minute and can only be studied under the 
highest powers of the microscope. The 
characters which define the species are 
particularly evasive because the forms are 
extremely simple in structure, and color 
and size are notoriously variable qualities 
among such organisms. Then these forms 
must be carefully compared with the in- 
habitants of other hot springs, particularly 
those of Germany and Italy, where such 
SCIENCE. 
[N. S. Vou. VI. No. 135. 
growths have been somewhat extensively 
studied. This is necessary in order to 
place the identifications upon a firm basis. 
Naturally, the most interesting forms 
from our point of view are those living in 
the hottest pools. The growth is usually 
white or gray; much of it has a pearly 
luster, and the structure is filamentous. In 
quiet water it forms a delicate network on 
the bottom. In the little streams which 
carry off the overflow from boiling pools 
one may find the filaments in tufts along 
the edge. Such tufts often appear as in 
Fig. 2, but the filaments may grow to be 
six inches long. The highest temperature 
of the water in which this growth has 
been so far reported is 85° C., life condi- 
tions which must, indeed, be considered re- 
markable. 
Examined under the microscope, the 
filaments present some curious features. 
Under the low powers one sees an appar- 
ently homogeneous strand of a gelatinous 
consistency, shown in Fig. 3, that is coated 
