PREPARATION OF SPECIMENS 3 



out the fixing fluid, although the advisability of stain- 

 ing at this point depends upon the type of stain em- 

 ployed and the kind of material one is working with. 



Dehydration. Paraffin or collodion may be used as 

 a supporting medium in the infiltration process, but 

 as both of these substances are immiscible with water, 

 dehydration or removal of water from the cells is essen- 

 tial. The removal of water from cellular material 

 must be accomplished gradually, and the water must 

 be replaced by liquids miscible with the paraffin or 

 collodion used as a supporting medium. Too rapid 

 replacement of water would result in injury to the cell 

 contents if not to the cell walls. As the process of 

 dehydration is dependent upon the exosmosis of water 

 and the endosmosis of the dehydrating fluid, the 

 rapidity of the process is governed by the permeability 

 of the cell walls. The dehydrating medium usually 

 employed in vegetable histology is ethyl alcohol. 

 After fixing and washing, the tissue fragments are 

 placed in alcohol and water mixtures of gradually 

 increasing concentrations, finally reaching absolute 

 alcohol. The length of time the specimen should 

 remain in each mixture depends upon the character of 

 the tissues, woody structures requiring more time than 

 those with soft membranous walls. In a specimen con- 

 taining various kinds of tissue one must adjust the time 

 of dehydration so as to secure full penetration of the 

 thicker walled tissues. As a specimen is not likely to 

 be injured by remaining too long in the alcohol and 

 water mixtures, it is better to give the maximum time 

 to each step in the process. 



Dried materials may also be prepared for sectioning 

 by the infiltration process, but in this case, fixing or 



