FREEHAND SECTIONS 97 



etc., may require a longer time and the acid may be used pure. Three 

 days in 10 per cent acid may be enough for corn stems. Wash thor- 

 oughly in water for a day or two. Then leave in equal parts of 30 per 

 cent alcohol and glycerin for several days before cutting. Material 

 may be left indefinitely in the mixture of glycerin and alcohol. 



An article by Dr. La Dema M. Langdon, dealing with the prepara- 

 tion and sectioning of hard, woody tissues, appeared in the Botanical 

 Gazette of July, 1920. By her method, hard, woody tissues are softened 

 so that they cut readily and can even be imbedded in paraffin and cut 

 successfully. 



OBJECTS MOUNTED WITHOUT SECTIONING 



Fern prothallia, mounted without sectioning, make very useful 

 preparations. Select desirable stages and fix in chromo-acetic acid for 

 24-48 hours (chromic acid, | g.; acetic acid, 3 c.c; water, 100 c.c). 

 Wash in running water 6 hours or overnight; stain in Delafield's hae- 

 matoxylin for 5-30 minutes; wash in water slightly acidulated with 

 hydrochloric acid for a few seconds, and then wash thoroughly in tap 

 water. The prothallia must now be brought through a graded series 

 of alcohols— 2i, 5, 7i 10, 20, 35, 50, 70, 85, 95, and 100 per cent. 

 About 2 hours should be the minimum in each grade, except 85, where 

 24 hours is better, since this is the best place to complete the harden- 

 ing. About 2 hours will be long enough for the 95 and 100 per cent. 

 Then use mixtures of alcohol and xylol. Then use a series of xylol- 

 alcohol, beginning with 5 parts xylol to 95 parts absolute alcohol. The 

 foUowing series seems to be close enough: 5, 10, 20, 35, 50, 75, pure 

 xylol. An hour in each may be sufficient. 



Then bring the prothallia into a mixture of xylol and balsam, using 

 at least 10 parts of xylol to 1 of balsam. If left in a shell, without cork- 

 ing, the xylol will soon evaporate, so that in a few days the prothallia 

 may be mounted. Use the balsam in which the material has been 

 standing, because any other balsam may have a different concentra- 

 tion. At every step in the process the prothallia should be examined 

 under a microscope, so that any shrinking may be detected. If each 

 succeeding step is tested with a single prothallium, a general disaster 

 may be avoided. If plasmolysis takes place, weaken the reagent and 

 try another prothallium. When a safe strength is found, bring on the 

 bulk of the material, and use the same method with succeeding steps. 

 The dangerous places are likely to be the transfer from alcohol to 



