22 
METHODS IN PLANT HISTOLOGY 
It will be found convenient to have in the laboratory the following 
stock solution of chromo-acetic acid from which various solutions 
can be made as they are needed: 
Chromic-acid crystals. 10 g. 
Glacial acetic acid. lOc.c. 
Water. 1,000 c.c. 
To make a solution containing 0.5 g. of chromic acid and 2 c.c. of 
glacial acetic acid to 100 c.c. of water, add 50 c.c. of water to 50 c.c. 
of the stock solution, and then add to the weakened solution 1.5 c.c. 
of glacial acetic acid. Any desired proportions can be secured in a 
similar way. Weighing the crystals for every new proportion is 
more tedious. The proportions of the various ingredients, for the 
present at least, must be determined by experiment. With favorable 
objects like fern prothallia, Spirogyra, and other things which can 
be watched while the fixing is taking place, suitable proportions are 
rather easily determined, because specimens, after being placed in 
the reagent, may be examined at frequent intervals, and combinations 
which cause plasmolysis may be rejected and different proportions 
tried until satisfactory results are secured. For example, fern pro¬ 
thallia might be placed in the following solution: chromic acid, 2 g.; 
acetic acid, 1 c.c.; and water, 97 c.c. If plasmolysis takes place, 
weaken the chromic or strengthen the acetic, since the chromic 
has a tendency to produce contraction, and the acetic to cause 
swelling. A 2 per cent solution of chromic acid is rather strong and 
a \ per cent is rather weak; for most things, 1 per cent seems to be 
about right. For the fern prothallia, the stock solution, with the 
addition of 2 c.c. of glacial acetic to 100 c.c. of the solution, is satis¬ 
factory for material to be mounted whole, and also for sections. 
A combination may be quite satisfactory for fern prothallia and still 
fail to give good results with Spirogyra, and a combination which 
succeeds very well with Spirogyra may not succeed at all with 
Vaucheria. For very critical work the most favorable proportions 
must be determined for the particular plant under investigation. 
In observing the effect of the fixing one can determine whether 
there is any noticeable plasmolysis or distortion, but whether the 
fixing is thorough can be determined only by noting how the tissues 
endure the subsequent processes. When the effect of the reagent 
cannot be observed directly, it is well to make a freehand section 
