18 PRACTICAL BOTANY. 



Hydrochloric acid. Used, with aniline chloride 

 phloroglucin, or carbolic acid, as a test for lignin. By 

 itself it turns lignified cell-walls yellow ; when its 

 action is prolonged, the cell-walls become violet, owing 

 to the presence of various substances such as phloro- 

 glucin, coniferin, and pyrocatechin. 



Chromic acid. A strong aqueous solution of this 

 acid dissolves lignified and cellulose cell- walls ; cuticu- 

 larised cell-walls resist its action ; but they become 

 very transparent, and may be easily overlooked. A 

 dilute solution brings out the stratification of cell-walls 

 very clearly. 



Acetic acid. This is used as a dilute aqueous 

 solution (1 per cent.). It dissolves crystals of calcium 

 carbonate ; it causes swelling-up of cell- walls, starch- 

 grains, &c. ; it brings out nuclei very clearly ; it is 

 useful as a corrective after treatment of a preparation 

 with potash. 



II. Alkalies. 



Potash. This may be used either in a dilute or a 

 concentrated solution in water. The dilute solution is 

 chiefly used for clearing preparations, as already de- 

 scribed. It causes cell-walls, starch-grains, &c., to 

 swell up very much, and it dissolves proteid crystalloids, 

 and most aleurone-grains. It gives a reddish colour 

 to cells in which tannin is present. It may be used as 

 a macerating fluid ; when woody tissues are boiled in 

 potash, the cells of the vascular bundles become more 

 or less isolated, for the lignin of their walls undergoes 

 solution. It dissolves inulin. 



The concentrated solution is used as a test for 

 suberin. When sections of cork are boiled in strong 



