460 THE ANATOMY OF WOODY PLANTS 



in cases where the structures present are to be reproduced by pho- 

 tographic methods. After the preparations have dried for a day 

 or two in a horizontal position at laboratory temperature, they are 

 weighted with lead in the form of rolls. For photomicrographic 

 purposes the final degree of flattening is secured by the pressure 

 of clip clothespins which are prevented from injuring the thin 

 covers by the interposition of thin slices of cork. In the processes 

 of flattening described above, the preparations are subjected to 

 continually increasing heat. This is conveniently secured by 

 laying the slides upon a board on top of a steam radiator. The 

 end of the board nearer the supply pipe of the radiator is naturally 

 much hotter than that nearer the return. The final flattening and 

 setting of the sections is best carried on in the interior of the par- 

 affin bath, which is an adjunct of every botanical laboratory. 

 It is unnecessary, of course, to resort to such extreme measures 

 to secure flatness unless the preparation is to be reproduced by 

 photomicrographic methods. 



In a number of instances, especially when the sculpture of the 

 cell wall in elements of the wood is a particular feature of interest, 

 mounting in Canada balsam brings about too great a degree of 

 homogeneity. This inconvenience can be avoided by using other 

 media for mounting. Gum dammar dissolved in chloroform sup- 

 plies a mountant which presents many of the advantages of Canada 

 balsam without the extreme degree of clarification which makes 

 that reagent in some cases undesirable. Glycerin jelly is an even 

 better mounting medium, but has the disadvantage of tending to 

 extract the safranin. This can be largely overcome, however, by 

 transferring the sections after staining to glycerin containing a 

 large amount of safranin. Under these circumstances the safranin 

 is not extracted from the sections. In mounting, the sections are 

 placed upon the slide and warmed over a burner to make the gly- 

 cerin quite fluid. This is then allowed to drain off as much as pos- 

 sible, and any remaining trace is removed by wiping round the 

 section with a piece of clean cotton fabric. A drop of melted gly- 

 cerin jelly is then applied and the cover is placed in position. 

 Weighting is advisable during setting and cooling of the jelly to 

 secure flattening of the sections. 



