REAGENTS 
19 
of our formulas are merely empirical, for very few botanists are 
expert chemists, and those who have the requisite knowledge of 
chemistry are interested in physiological problems rather than in 
microtechnic. The principal ingredients of the usual killing and 
fixing agents are: alcohol, chloroform, chromic acid, bichromate of 
potash, potassium iodide, copper acetate, acetic acid, osmic acid, 
formic acid, picric acid, sulphuric acid, platinum chloride, iridium 
chloride, corrosive sublimate, and formalin. We shall consider first: 
THE ALCOHOLS 
a) Ninety-five Per Cent Alcohol. — This is in quite general use 
for material which is needed only for rough work. It is extremely 
convenient, since it kills, fixes, and preserves at the same time and 
needs no changing or washing. It really has nothing to recommend 
it for fine work. It causes protoplasm to shrink, but cell walls 
usually retain their position, so that 95 per cent alcohol will do for 
freehand sections of wood and many herbaceous stems, where it is 
not necessary to preserve cell contents; but even freehand sections 
of tender stems, like geraniums and begonias, will look better if 
better reagents are employed. Alcohols weaker than 95 per cent 
are not to be recommended as fixing agents, although 70 per cent al¬ 
cohol, or even 50 per cent, will preserve material for habit work. 
The time required for fixing in 95 per cent alcohol is about the 
same as for absolute alcohol. The subsequent treatment is the same, 
except that material to be imbedded in paraffin or celloidin must 
be dehydrated in absolute alcohol. Material preserved in weaker 
alcohols and intended only for habit study may be kept in the reagent 
until needed for use. Unless some glycerin be added, material left in 
95 per cent alcohol becomes very brittle. Stems, roots, and similar 
objects may be kept indefinitely in a mixture of equal parts of 95 per 
cent alcohol and glycerin. 
Methyl alcohol, or wood alcohol as it is commonly called, serves 
equally well. 
b) Absolute (100 Per Cent) Alcohol.—This is a fair killing and 
fixing agent, it causes but little shrinking of the protoplasm, and is 
a time-saver if material is to be imbedded in paraffin. The time 
required for fixing in alcohol is very short. For small fungi, like 
Eurotium, 1 minute is long enough. Root-tips of the onion, anthers 
of the lily, and similar objects require 15 to 30 minutes. Larger 
