30 



HISTOLOGY OF MEDICINAL PLANTS 



that they did in the living plant, and to fix the parts so killed. 



After the fixing process is complete, the specimen is freed 

 of the fixing agent by washing in water. From the water-bath 

 the specimens are transferred successively to 10, 20, 40, 60, 70, 

 80, 90, and finally 100 per cent alcohol. In this 100 per cent 

 alcohol-bath the last traces of moisture are removed. The 



Fig. 31. — Paraffin-embedding Oven 



length of time required to leave the specimens in the different 

 percentages of alcohols varies from a few minutes to twenty- 

 four hours, depending upon the size and the nature of the 

 specimen. 



After dehydration the specimen is placed in a clearing agent 

 — chloroform or xylol — both of which are suitable when em- 

 bedding in paraffin. The clearing agents replace the alcohol in 

 the cells, and at the same time render the tissues transparent. 

 From the clearing agent the specimen is placed in a weak solu- 

 tion of paraffin, dissolved xylol, or chloroform. The strength of 

 the paraffin solution is gradually increased until it consists 

 of pure paraffin. The temperature of the paraffin-embedding 



