BOTANICAL TECHNIQUE 621 



solutions of corrosive sublimate or picric acid give better results. 

 For objects which easily turn black, Overton {Zeit. wiss. Mik., 

 vii, 1890, p. 9) uses alcohol containing sulphurous acid. An 

 aqueous or alcoholic solution of picric acid may also be combined 

 with sulphurous acid. 



Warrington {Ann. Bot., xl, 1926, p. 27) uses a mixture of 

 3 grm. corrosive sublimate, 3 c.c. glacial acetic acid and 100 c.c. 

 70 per cent, alcohol, afterwards washing in 70 per cent, alcohol 

 until iodine is no longer decolourised. Such thorough washing 

 must follow all corrosive sublimate fixations. More delicate 

 materials require better fixation. The following widely used 

 fixatives are roughly in order of excellence : Flemming and its 

 modifications (§ 1356), Navashin (§ 1357), chromo-acetic (§ 44), 

 Bouin (§ 1358), aceto-formalin alcohol (§ 1235), formalin alcohol 

 (§ 1235). For greater precision of action smaller pieces of material 

 should be used ; in Flemming fluids the diameter should not 

 exceed 3 to 4 mm., and is preferably much less. No effort should 

 be spared to expose the critical cells and tissues to the direct action 

 of the fixative. Except with formalin, alcohol and formalin 

 alcohol (which are also preservatives), the material should be 

 washed well and brought gradually into 70 per cent, alcohol, in 

 which it is stored. 



See also Cobb, Trans. Am. Micr. Soc, xlvi, 1927, p. 153 (thermo- 

 lethe for hot fixatives) ; Gates Science, xxxi, 1910, p. 234. 

 Marine organisms should be fixed in fluids made up with sea- 

 water ; see § 1240 and § 1378. 



1235. Preservatives. For general purposes, 70 per cent, alcohol 

 is the safest ; add 5 to 20 per cent, of glycerin (Calberla's fluid) 

 to minimise the effects of evaporation. Reduce the alcohol 

 concentration to 50 per cent, for woody or tough inaterial, and 

 increase to 85 per cent, for delicate material. Formalin, at 4 per 

 cent, strength, keeps material in a less brittle state than strong 

 alcohol. Calcified specimens should be kept in alcohol or neutral 

 formalin. Prepare the latter by adding borax until the solution 

 gives a red colour with phenolphthalein. 



Methyl alcohol may be substituted for ethyl, but denatured 

 alcohol should be avoided owing to the cloudiness of the aqueous 

 solutions. 



Other preservatives : 96 c.c. of 70 per cent, alcohol and 4 c.c. 

 of formalin ;. 100 c.c. of 50 per cent, alcohol, 6-5 c.c. formalin and 

 2-5 c.c. glacial acetic acid. 



See also §§ 1286 bis, 1378 and 1393. 



1236. Preservation of Special Substances. Calcium Carbonate or 

 Sulphate Deposits. Fix in alcohol free from acids. Cystoliths 

 should be stained, rapidly dehydrated, cleared and mounted in 

 balsam. A small degree of solution is diflicult to avoid ; cystoliths 

 should not be left in tap-water, in which they are often soluble. 



