16 THE ELEMENTS OF VEGETABLE HISTOLOGY 



ing treatment. Hardening may be accomplished by 

 immersion of small pieces of the material in 6 per 

 cent formalin solution or in 50 per cent alcohol. The 

 length of time required for hardening varies accord- 

 ing to the texture of the material. Prolonged harden- 

 ing is to be avoided, as it tends to make the specimen 

 brittle and is apt to cause distortion of the cellular 

 elements. 



Dried Materials. — Dried or hard materials should 

 be softened before blocking. Softening may be accom- 

 plished by soaking the specimen in a mixture of gly- 

 cerin, alcohol, and water (1 : 1 : 1), or by boiling in 

 water. While the glycerin mixture is slower in action 

 it has an advantage in that with it there is little danger 

 of rendering the specimen too soft. Gentle heating 

 will hasten the process. With materials of excep- 

 tional hardness boiling is necessary, and small amounts 

 of acid or alkali may be added to aid in the softemng. 



Blocking for Cutting. — As most objects are too 

 small to be conveniently held during sectioning, it 

 is necessary to use a supporting medium. The speci- 

 men, if fairly soft, may be placed between slices of 

 fresh potato or carrot and thus held firmly enough 

 for sectioning. The objection to this method of 

 blocking is that it is often difficult to separate small 

 portions of the blocking material from the sections, 

 especially if the latter are of small size. Pieces of 

 elder pith may be used as a holding medium for mate- 

 rials of soft texture. Paraffin is the best holding 

 medium for non-infiltrated specimens, as it may be 

 used with materials of almost any texture. Further- 

 more it can be readily separated from the sections, 

 and by merely dipping the cut surfaces of the block 



