ANATOMICAL TECHNIQUE 447 



is naturally reached by the use of aqua regia (nitro-hydrochloric 

 acid) or of nitric acid and a chlorate. 



THE SOFTENING OF MATERIAL 



In the case of the hard tissues of plants it is usually necessary 

 to employ some method of softening in order to facilitate the sub- 

 sequent cutting of sections. It has been discovered that the hard- 

 ness of the wall of the cell in plants, other things being equal, 

 depends upon the amount of mineral matter present. Lime, 

 aluminum, and silicon are elements often found in the wall, par- 

 ticularly where it is considerably thickened. The removal of 

 these substances depends, of course, upon the use of an appropriate 

 acid, and hydrofluoric acid has proved most useful in this connec- 

 tion. This acid has the advantage of not attacking the middle 

 lamella an advantage which is of considerable importance where 

 good sections have to be secured. For most purposes strong, 

 commercial hydrofluoric acid answers sufficiently well, but in 

 cases where the tissues are delicate the chemically pure reagent 

 is best employed. Hydrofluoric acid is best purchased in gallon 

 leads containing about ten pounds in weight. The leads should be 

 massive, as thin ones are sometimes eaten through, with disastrous 

 results, before the acid is used up. The material is prepared for 

 treatment in different ways, depending on its nature. In the case 

 of the shell of a hickory nut or a piece of ebony or live oak, the dry 

 material is first boiled for some time in water. The boiling should 

 be continued after the sinking of the material so as to insure the 

 driving out of all air and the complete imbibition of the mem- 

 branes. It is then allowed to cool and is transferred to hydro- 

 fluoric acid of full strength. After sojourn in the reagent for a 

 week or two a piece is washed for a short time in running water 

 and is then tested with a knife to discover if it cuts readily. If 

 not, a longer stay in acid is indicated, and in case of very hard sub- 

 stances the time of sojourn may be as long as five or six weeks with 

 occasional renewal of the reagent in extreme instances. The hard- 

 est and most refractory tissues of existing plants may be mastered 

 in this manner. Wood is usually cut into cubes, which may advan- 

 tageously be about one centimeter in each of the three dimensions 



