38 METHODS IN PLANT HISTOLOGY 



Xylol. — In our opinion, xylol is the best clearing agent to precede 

 infiltration with paraffin. After the material has been dehydrated, it 

 should be brought gradually into xylol. Thirty years ago it was cus- 

 tomary to bring material directly from absolute alcohol into xylol; 

 twenty years ago, two or three mixtures of absolute alcohol and xylol 

 were used before reaching the pure xylol; at present, those who are do- 

 ing the most critical work are making this process still more gradual. 

 As cytologists have been studying more and more minute structures, 

 the methods have become more and more critical. As in the case of the 

 alcohol series, the xylol series has its grades closer together at the be- 

 ginning than at the end. The following series seems to be sufficiently 

 gradual: -jV, h I, 2; f; P^re xylol. It is hardly necessary to use a 

 graduate in making up the series. For the |, use equal parts of xylol and 

 absolute alcohol; for the |, use equal parts of the | and absolute alco- 

 hol; for the I, use equal parts of the | and absolute, and for the iV> 

 equal parts of the | and absolute. The f can be guessed at with suffi- 

 cient accuracy. 



We prefer a closer series of xylols, using 2|, 5, 7|, 10, 15, 20, 30, 40, 

 50, 75, and 100 per cent. Infiltration with paraffin is more thorough 

 with this closer series. We use it and recommend it. Three grades a 

 day, morning, noon, and night, will do for filamentous algae and fungi, 

 fern prothallia, onion root-tips, and similar objects. For larger pieces 

 the times should be longer. For |-inch cubes, change morning and 

 evening. For still larger pieces, 24 hours in each grade is not too long. 

 In all cases, the pure xylol should be changed 2 or 3 times. While the 

 pure xylol must not be used again for this purpose, it is still good for 

 dissolving paraffin ribbons when staining on the slide. 



Xylol is the best agent for clearing sections just before mounting in 

 balsam. Preparations cleared in xylol harden more rapidly, and this is 

 such a decided advantage that even when sections have been cleared in 

 cedar oil or clove oil it is worth while to give them a minute or two in 

 xylol before mounting. Besides, clove oil is a solvent of many of the 

 most frequently used stains and, consequently, preparations in such 

 stains would fade, if transferred directly from clove oil to balsam. 



Xylol evaporates so rapidly that one must take care not to let sec- 

 tions become dry before applying the balsam. Thin sections perfectly 

 dehydrated seem to clear in a few seconds; but, even with very thin sec- 

 tions, it is better to let the xylol act for at least a minute. Sections 20 n in 

 thickness should remain in the xylol 5 minutes before mounting in bal- 



