130 The Preparation of Microscope Slides 



as the oils are nonvolatile, there is no chance of getting rid of them in the 

 embedding oven. 



Dehydrating and Clearing Plant Tissues. As was pointed out in 

 Chapter 8, the methods of clearing and dehydrating animal tissues cannot 

 satisfactorily be applied to plant tissues because they cause the cell con- 

 tents to pull away from the cell walls. Many methods have been sug- 

 gested but the majority of botanists today prefer the "Zirkle technique." 

 A schedule of this type, which has been found satisfactory, but which 

 workers may wish to modify from their own experience, involves passing 

 the tissues through the following mixtures: 



10. Tertiary butyl alcohol 



The length of time that the tissues remain in each stage depends on 

 their size but should be not less than 1 hr nor more than 6 hr in the 

 first five fluids. The sixth mixture is the critical point and most people 

 leave even small tissues in it overnight. One hour in each of the remain- 

 ing fluids is usually enough for medium-size pieces. 



Choice of an Embedding Medium. It is to be presumed at the present 

 time that no one will endeavor to use a plain paraffin but will use a mix- 

 ture. If a plain paraffin is preferred, then it is necessary to buy (in the 

 United States by importation ) a carefully fractionated and very expensive 

 wax. Ordinary cheap paraffin is a mixture of a great variety of compounds 

 of slightly different melting points; it is essential in the use of pure wax 

 that a wax of a very sharp melting point be obtained. 



The choice of an embedding medium should be dictated less by the 

 nature of the specimen than by the conditions under which it should be 

 cut. If pure paraffin is to be employed, it should be of such a melting 

 point as will give the hardened wax a crisp section at the required room 

 temperature. Since the introduction of any foreign substance automatically 

 lowers the melting point of the wax, it is obviously desirable to use mix- 

 tures rather than the pure material. For ordinary routine preparations, the 



