CHLOROPHYCEAE 
179 
For fixing, Professor Powers uses the aqueous potassium iodide 
solution used in testing for starch. This solution may be weaker 
than the usual formula so that it has a light-brown color. From 
10 to 24 hours is sufficient for fixing, but material may be left here 
for several days. Wash thoroughly in tap water which has stood 
long enough to give off all of its excess of air; otherwise bubbles 
will form on the colonies, causing them to float and hindering 
subsequent processes. Change after change of water should be 
made rapidly, using large 
amounts of water and 
decanting just as soon as 
the colonies have settled. 
From 1 to 3 hours’ wash¬ 
ing should be sufficient to 
remove the brown color 
of the iodine. 
Stain in Mayer’s Car- 
malum. 1 Use a pure car- 
minic acid in making the 
stain, 1 g. carminic acid, 
10 g. alum, and 2Q0 c.c. 
distilled water. Dissolve 
with heat, filter, and add 
a crystal of thymol to 
keep out fungi. After 
staining, follow the Vene¬ 
tian turpentine method material fixed in chromo-acetic acid and stained in Dela- 
^ } field’s haematoxylin; from a preparation and negative by 
taking care to wash the Dr. w. j. g. Land, 
glycerin out completely. 
The 10 per cent turpentine should not be allowed to concentrate too 
rapidly. 
Material fixed in weak osmic acid is even better for protoplasmic 
connections and cilia. About 4 or 5 drops in 50 c.c. of distilled water 
is sufficient. From 6 to 24 hours is long enough for fixing. 
After either of these fixing agents, following the washing in water, 
material may be preserved in a nearly saturated solution of alum; 
or in a dilute aqueous carmalum, with a crystal of thymol to prevent 
mold. Staining for 3 weeks in a weak carmalum stains the cells 
1 Lee Vade-Mecum (8th ed. p. 137). 
Fig. 37.— Volvox: photomicrograph of a section from 
