120 METHODS IN PLANT HISTOLOGY 



quire 6 or 8 hours; if serial sections through the entire cone are 

 wanted, Dr. Hannah Aase found that the time must be prolonged to 

 24 or even 48 hours. Some particularly difficult material which will be 

 mentioned in the chapter on "Special Methods," may require several 

 days. 



When one is dealing with many lots of the same kind of material, as 

 in research work, the time required for infiltration is easily determined. 

 As a rule, minimize heat. It is, probably, never necessary to use paraf- 

 fin with a melting-point higher than 52° C. With Land's cooling device 

 sections 1 ^ in thickness can be cut from 52° C. paraffin. 



A few final hints may not be amiss. If the dehydration is not com- 

 plete, it is practically impossible to replace the alcohol completely with 

 xylol. Unless this replacement is complete, the infiltration with paraf- 

 fin will be imperfect. Students are likely to prolong the time in the 

 paraffin bath in a vain attempt to force paraffin into a tissue which 

 still contains some xylol. When the alcohol series and xylol series have 

 done their work perfectly, the time in the bath is likely to be shorter 

 than most investigators allow for this stage. 



IMBEDDING 



Material may be imbedded in paper trays or any apparatus made for 

 the purpose. A satisfactory imbedding-dish is a thin rectangular porce- 

 lain dish glazed inside. This dish, called a Verbrennungsschale, is 

 made by the Konigliche Porzellan-Manufactur, Berlin, Germany. The 

 most convenient sizes are 40X50X10 mm., 68X45X10 mm., and 

 91 X 58 X 15 mm. As listed, these dishes are not glazed; care should be 

 taken to indicate that the dishes must be glazed inside {innen glasiert). 

 The paper tray, if well made, is as good as anything. Thick ledger 

 linen or thin, smooth cardboard makes good trays. 



Smear the dish or tray with glycerin or soapy water to prevent 

 sticking. Another way to prevent sticking is to put a piece of tissue 

 paper in the dish, pour on water and make the tissue paper fit the 

 inside of the dish, and then pour on the paraffin with the material to be 

 imbedded. The paraffin will not stick to the paper. If several objects 

 are to be imbedded in one dish, it is best to have the dish as near the 

 temperature of melted paraffin as possible; otherwise, the objects may 

 stick to the bottom, and it will be impossible to arrange them properly. 

 Hot needles are good for arranging material. Great care should be 

 taken not to have the dish too hot, since too high a temperature not 



