1905] PEIRCE & RANDOLPH—I]RRITABILITY IN ALGAE 323 
floating near the surface of the water in the troughs. This water 
was quiet and fairly clear. The level of the water, tending to be 
lowered by evaporation or by the draughts of the few horses using 
the troughs, was automatically maintained by ball-valves, similar to 
those used in houses. A few other plants, apparently of the same 
species, were found in a quiet pool in the San Francisquito Creek, 
growing on stones in comparatively clear water. 
The material was brought fresh into the laboratory from time to 
time from mid-September till mid-November, and again from early 
April till late in May. Since the material did not fruit in our cultures, 
and we found none fruiting out of doors, it was impossible to determine 
the species, though we tried to have this done for us. We regret this 
lack of definiteness in our work. 
METHOD OF MANIPULATION. 
Plants brought into the laboratory were placed in an abundance 
of tap-water in glass dishes covered with a glass plate or loose cap to 
Prevent excessive evaporation and to exclude dust. Small Stender 
dishes were largely used, since these could easily be placed under the 
Microscope, thereby avoiding such disturbance of the material as 
transfer from one dish to another would entail. 
Fortunately for us the water from the tap was from the same source 
that supplied the horse-troughs, and the creek contains mainly the 
overflow from the artificial lake which is the general water-supply 
for the region, When, therefore, our plants were brought in and put 
Into tap-water in the laboratory, they were put into water of the same 
Composition and approximately of the same temperature and degree 
of aeration as the water of the troughs and the pool from which they 
been removed. The behavior of our plants was immediately 
affected only by those factors changed by the transfer to the laboratory, 
pe y the —— , though it goes without saying that the temperatures 
t © cultures in the laboratory never fell quite so low at night during 
the winter and spring as that of the water outside in troughs and pools. 
: ultures were also made in Knop’s solution,” 1 per cent. and 
a Per cent. In certain cases we added ro per cent. gelatine or 
‘25 Per cent. agar-agar to the Knop’s solution, in order to obtain a 
t _, osoligeeSegegs I part potassium nitrate, 1 part magnesium sulphate, 
. 
