BOTANICAL TECHNIQUE 625 



allowed to concentrate, but its action is too harsh except for 

 clearing whole inistained mounts. 



Overton's method for microscopic objects {Zeit. wiss. Mik., 

 vii, 1890, p. 9), After fixation in a drop on a slide or eoverslip, 

 add a drop of 10 to 20 jjcr cent, alcohol and support the prepara- 

 tion above absolute alcohol in a close chamber. A shallow dish 

 with its upper edge ground and sealed with a vaselined glass plate 

 is an efficient container. The chamber must be set in an even 

 temperature, away from insolation. In a few hours the drop 

 becomes almost absolute. Then add a drop of very dilute alcohol- 

 ether-eelloidin solution (about one-fifth the strength of the first 

 solution used for celloidin imbedding) and spread it evenly by 

 tipping the preparation back and forth. As soon as the celloidin 

 no longer appears to flow, immerse the slide in 80 per cent, alcohol, 

 wet side up. The film becomes hard in a few minutes and the 

 material can be further handled without fear of loss. 



See also Metz, Anat. Rec, xxi, 1921, p. 373 ; Allen, E., 

 Science, Ixvi, 1927, p. 427 ; Radir, Science, Ixxi, 1930, p. 613. 



1241. Clearing of Bulk Material. Any glycerin used in the 

 preservative or for dehydration must first be removed by repeated 

 changes of alcohol. An alcohol-xylol series is most used for 

 clearing, but alcohol-chloroform seems preferable. Cedar oil is 

 good ; place some in a phial and an equal amount of absolute 

 alcohol containing the material above it. The objects sink 

 through into the oil and become impregnated ; wash in clean oil 

 and replace with xylol. 



Hartridge uses No. 1 petrol (see § 1240). 



1242. Clearing of Material for Mounting. Xylol is most used ; 

 a graded alcohol-xylol series is indicated if material tends to 

 collapse. Clove oil is very useful, it will clear sections from 

 95 per cent, alcohol and is frequently used as a differentiating 

 agent ; wash out well with xylol afterwards. Anilin oil is 

 hard on stains and cedar oil easily clouds with atmospheric 

 moisture. 



With celloidin sections a mixture of 25 per cent, or less crystal- 

 lised carbolic acid and 75 per cent, or more xylol is probably 

 best ; Eyclcshymer's fluid sometimes injures the stain. 



Other Methods. In §§ 124, 133 are given other methods 

 used in animal microtomy and also applicable to plant material. 



1243. Imbedding. For a general account of botanical methods, 

 see Kisser {Abderhalden Handb. biol. Arbeitsmetlioden, Abt., xi, 

 Teil 4, pp. 391, and 533). The use of paraffin is advised wherever 

 possible ; that of collodion or other media only where they are 

 imperative. 



1244. Paraffin Method. See Kisser (Cytologia, iv, 1933, 

 p. 288) for a critical survey. The solvents in general use are 

 xylol and chloroform. Plant material requires more time than 



