PALEOBOTANY 719 



surface. Apply a thin layer of clove oil to the smooth surface 

 and press it, either cold or slightly warm, on a heated glass slide, 

 using a mixture of balsam and gum copal for cement. Cool the 

 block and leave it until it is hrmly cemented to the slide. Place 

 it in a vice and cut it through with a hack-saw, leaving a thin 

 slice fastened to the slide. Grind this slice to the desired thickness, 

 dry and add balsam and a coverslip. Seyler and Edwards 

 (The Microscopical Examination of Coal, H.M. Stationery Office, 

 London) give methods for cutting thin sections and a method, 

 based on metallographic practice, of polishing and etching flat 

 surfaces to bring the plant structures into prominence. Turner 

 {Amer. Inst. Min. and Met. Eng. Advance Paper, No. 1409-1, 

 1925) polishes small blocks of anthracite, etches them with heat 

 and examines them with reflected light using a metallographic 

 microscope. Heard {Quart. J. Geol. Soc, Ixxxiii, 1927, p. 195) 

 imbedded pyritised specimens, from the Old Red Sandstone, in 

 shellac and carefully ground and polished them with the finest 

 carborundum flour. The polished sections were then treated 

 with concentrated nitric acid to bring out the internal structure 

 of the plants. 



The differentiation of cell-wall and other details in fossilised 

 specimens usually depends on the presence of carbonised material 

 representing the plant, imbedded in some matrix, usuafly sili- 

 ceous or calcareous. Sections in which there is insufficient differen- 

 tiation may be much improved by Kisser's Anthracogram method 

 {J. Indian Bot. Soc, x, 1931, p. 60). Pass them through alcohol 

 and benzol and boil them in liquid paraffin under a large cover- 

 glass for one minute or more. Traces of organic matter in the 

 walls are carbonised and appear first yellowish, then reddish- 

 brown and finally blackish. When the desired degree of car- 

 bonisation has been reached, cool the slide, dissolve away the 

 paraffin and mount in the usual way. 



Semi-fossilised peats, soft brown coals and partially silicified 

 woods may be desilicified with hydrofluoric acid and sectioned 

 on a sliding microtome w^ith or without imbedding in celloidin. 

 Lang [Ann. Bot., xliii, 1929, p. 663) dehydrates and imbeds in 

 paraffin wax from chloroform ; sections can then be obtained in 

 ribbons. 



Jeffrey {Anatomy of Woody Plants) recommends that the 

 material be soaked in carbolic acid under pressure in a wire- 

 stoppered bottle in a paraffin oven both before and after treat- 

 ment with hydrofluoric acid. Tlic periods of immersion in each 

 reagent should be about one week, and the treatments should be 

 repeated if necessary. 



Peel Methods of Sectioning Petrifications were originally due to 

 Walton {Nature, cxxii, 1928, j). 571): see also Nature, cxxv, 

 1930, p. 1413. Grind a flat surface on the petrified mass, parallel 



