BOTANICAL TECHNIQUE 623 



1239. Herbarium material (or dried crude vegetable drugs) for 

 anatomical study, cither following imbedding and sectioning or 

 maceration, should first be moistened with alcohol to reduce the 

 air film and then soaked in water. Gentle heat may be applied 

 to hasten the j)roccss, and a vacuum pump should be used to free 

 the material from all air. Various methods have been adopted for 

 softening and clearing such material. 



Lagerheim [Hedivigia, 1888, p. 58 ; Rev. MijcoL, xi, 1889, 

 p. 95) softens dried algae and fungi in water and then gently warms 

 them in concentrated lactic acid imtil they show small bubbles. 

 This method is less certain with other, more resistant, material. 

 The addition of })henol, recommended by Zimmermann, is of 

 little help on old material, the gel colloids of the walls having 

 lost part of their reversibility. Lacto-phenol and lacto-glycerin 

 have also been used. 



McLean's method {New Phyt., xv, 1916, p. 103). Place the 

 material directly mto absolute alcohol for at least twenty-four 

 hours. Pieces of large size should also be subjected to a reduced 

 pressure (10 cm. mercury or less) during their immersion ; delicate 

 and small objects do not require this treatment. Next, grade down 

 very gradually to distilled water, in which the material may be 

 left indefinitely. The growth of moulds is prevented and recovery 

 hastened by placing in a hot chamber {e.g., paraffin oven). 

 Material in bulk soaks out better than sections. When the 

 material is well soaked, transfer to 4 to 8 per cent, aqueous potash 

 for six to nine days. Better results are obtained if, after half the 

 period of immersion is over, the potash is allowed to concentrate 

 to one-third its original volume. The potash must not be heated, 

 otherwise the cellulose tissues would be destroyed. A reduced 

 pressure assists removal of air and is to be employed during the 

 concentration of the potash. Neutralise the potash with several 

 changes of dilute acetic acid (15 to 20 per cent.) ; avoid the use 

 of mineral acids. Then wash in water until the latter is neutral 

 to litmus. If the material is still deeply coloured, the acetic acid 

 may be shaken up first with bleaching powder, excess of which 

 is removed by filtration. 



The possibilities of the method are limited by the method of 

 drying. The more thorough and rapid has been the drying, the 

 better is the resuscitation. 



See also Arber, Ann. Bat., xl, 1926, p. 447 ; ibid., xliii, 1929, p. 41. 



Staining presents some difficulties, since the solvent action of 

 potash on lignin affects the reactions of lignified walls. McLean 

 recommends a 2 per cent, aqueous solution of fuchsin or a solution 

 of fuchsin decolourised by sulphurous acid for ten to fifteen 

 minutes. After the latter stain, wash in tap-water until excess 

 acid is removed. Counterstain with light green in clove oil after 

 dehydration. 



