118 METHODS IN PLANT HISTOLOGY 



material, but the block of paraffin should not rest directly upon the 

 objects. Dr. Land uses coarse wire gauze, cut into strips about 15 mm. 

 wide and tapered at both ends. The strip is then bent so that the 

 pointed ends rest upon the bottom of the Number 4 shell, while the 

 middle portion forms a flat table upon which the paraffin may rest. 

 Dip the wire gauze table into xylol and then slip it carefully into the 

 Number 4 shell. The table portion should be 10 or 15 mm. above the 

 material, and there should be enough xylol to extend a few millimeters 

 above the table. Place on the table a block of paraffin about equal to 

 the volume of the xylol in the shell. The table not only prevents the 

 paraffin from injuring the material by mechanical pressure but insures 

 considerable diffusion before the mixture of paraffin and xylol reaches 

 the specimens. After 24 hours (or several days, if time permits) at 

 room temperature, place the shell on a pasteboard box — slide boxes are 

 good — on top of the paraffin bath. Do not place the shell directly upon 

 the metal of the bath, since it is better to minimize heat. As soon as 

 the paraffin is dissolved, add some more, this time leaving the cork out, 

 in order that the xylol may evaporate. About 24 hours on the top of 

 the bath should be sufficient. 



THE PARAFFIN BATH 



This step is usually called infiltration, but when the transfer from 

 the clearing fluid to paraffin is made gradually, as has just been indi- 

 cated, the process of infiltration is already begun. It is now necessary 

 to get rid of the xylol or other clearing agent. This is accomplished, 

 to a considerable extent, by pouring off the mixture of xylol and paraf- 

 fin and replacing it with pure melted paraffin. Pour off the pure 

 paraffin immediately. This is important. You will notice that often, 

 when the pure paraffin is poured on, a froth or scum will appear on the 

 surface. Much of the xylol will be in this scum, and, if allowed to re- 

 ma" n, it would diffuse into the mass and greatly prolong the time needed 

 for infiltration. So, pour it off and add more pure paraffin, for some 

 xylol remains in the tissues and must be removed. Dot not put the 

 shell into the bath, but use a flat dish of some sort. The main object is 

 to have a fairly large surface exposed, so that the remaining xylol may 

 evaporate as rapidly as possible. Change the paraffin 3 or 4 times. A 

 good 52° C. paraffin will yield smooth sections from | /x up to 20 n. 

 Where thicker sections are needed, a 45° C. paraffin should be used. 

 For many years we have used only a 52° C. paraffin. 



