152 
ner that the delicate thread travels over 
its substratum and among other alge. 
Spirulina mixed with Phormidium often 
forms curious raised rims about the pools 
of hot water. This phenomenon is es- 
pecially well shown upon the edge of the 
Prismatic Spring in the Middle Geyser 
Basin, also known under the more pictur- 
esque name of Hell’s Half Acre. The 
Prismatic Spring is an immense pool of 
hot water about a hundred yards wide. 
The center of the spring is of a dark blue 
color, which gradually changes through 
shades of green to a light yellow around 
the margin, where the water is shallow 
over the sloping bottom of the basin. On 
the edge of the pool is a greenish growth 
variegated with brown and yellow. It has 
the form of a rim several inches wide 
raised above the mineral substratum and 
acting as a slight obstruction to the ripples 
of hot water that constantly splash over 
it upon the formation. The substance of 
the growth is rather firm like wet felt, and 
the surface, figured with raised lines, re- 
sembles tripe. The masses of wet alge 
are so warm that one cannot hold them, 
and the water that laps the edges of the 
deposit is much too hot to approach with 
safety. It is in situations such as these 
that the alge of thermal springs play an 
important part in moulding the form of the 
mineral deposits, but this is a subject that 
will engage our attention later on in the 
paper. 
We have now considered the organisms 
that are most characteristic of the hot water 
of the thermal springs. It is altogether 
probable that careful studies on the spot 
would bring out many interesting facts in 
respect to the number of species, and the 
precise range of their distribution through 
the various springs in relation to the tem- 
perature and character of the water. It is 
an interesting field for study, and deserves 
the attention of a botanist for three months, 
SCIENCE. 
[N. 8. Von. VI. No. 135. 
instead of three days, which the writer once 
spent in the Geyser Basin. 
As the water grows cooler in the over- 
flow streams the conditions gradually be- 
come suitable for types more familiar to 
the botanist. A species of Anabena (Fig. 
4—f) is prominent in some places, and mem- 
bers of the Chlorophyceze and diatoms are 
found, but these types are not truly a part 
of the flora of the thermal springs. 
One more form deserves notice from the 
peculiarity of its structure and the curious 
situations in which it is found. There are 
places in the Geyser Basins and on the de- 
posits of Mammoth Hot Springs where 
steam issues from the cracks and crevices 
of the formation. One may frequently find 
the mineral matter in these openings col- 
ored a bright green by a delicate film of 
slime. This is largely made up of Glao- 
capsa (Fig. 4-g), a unicellular alga that is 
characterized by the presence of very thick 
cell walls made up of concentric layers, the 
outer becoming gelatinous. These slimy 
films must be kept continually damp by the 
hot steam that issues from the vents. 
The inhabitants of the thermal springs 
are all members of one or two families of 
plants, and a greater interest attaches to 
them because the fact is plain that the con- 
ditions under which they live are not suit- 
able for the forms of alge that commonly 
crowd our pondsand streams. Ina general 
way they are all closely related to one 
another. The colored forms are all mem- 
bers of the class Cyanophycez, the lowest 
group of the algze, and the colorless type all 
belong to the class Schizomycetes, or bac- 
teria, the lowest group of the fungi. More- 
over, the forms of Cyanophyces and Schizo- 
mycetes are quite generally considered to 
be near relatives. Indeed, all the members 
of both groups have been classified under 
one name by some authors and called 
Schizophytes, which means plants that split. 
All the forms agree in having a very simple 
