REAGENTS 19 



Zoologists often use chloroform or ether for killing an organism, and 

 then use various fixing agents for various tissues. 



Most of our formulas are merely empirical, for very few botanists 

 are expert chemists, and those who have some knowledge of chemistry 

 are interested in physiological problems rather than in microtechnique. 



The principal ingredients of the usual killing and fixing agents are : 

 alcohol, chloroform, chromic acid, dichromate of potash, potassium 

 iocUde, copper acetate, acetic acid, osmic acid, formic acid, picric acid, 

 sulphuric acid, platinum chloride, iridium chloride, corrosive subH- 

 mate, and formalin. We shall consider first : 



THE ALCOHOLS 



a) Ninety-five per cent alcohol. — This is in quite general use for 

 material wliich is needed only for rough work. It is extremely con- 

 venient, 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 pre- 

 serve cell contents; but even freehand sections of tender stems, like 

 geraniums and begonias, will look better if better reagents are em- 

 ployed. Alcohols weaker than 95 per cent are not to be recommended 

 as fixing agents, although 70 per cent alcohol, 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 subse- 

 quent 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 shrinldng 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, hke Eurotium, 1 minute 



