MICRO-CHEMISTRY OF THE CELL. 31 



c. Cuticularised cell- walls (including cork) 



i. Coloured yellow by iodine, by Schulze's solution, 

 and by iodine and sulphuric acid. 



ii. Coloured yellowish by concentrated solution of 

 potash ; on gradually warming (without boiling), it 

 becomes bright yellow ; on boiling, yellow drops of 

 suberin escape. 



iiii. On treatment with Schulze's macerating fluid, the 

 cuticularised cell-walls become conspicuous ; on boiling, 

 viscous drops (impure suberic acid) escape, which are 

 soluble in hot alcohol, ether, benzol, chloroform, and 

 dilute potash solution. Traces of cuticularisation may 

 be detected by treating the tissue for a short time with 

 Schulze's fluid without heating, and then with potash ; 

 the cuticularised cell-walls become conspicuous and turn 

 yellow ; the colour may be intensified by gently warm- 

 ing in potash. 



iv. Dissolved very slowly in concentrated chromic 

 acid ; hence on treatment of a section with this reagent 

 the cuticularised cells are the last to disappear. 



v. Not stained by solutions of carmine or hsematoxy- 

 lin ; stained by aniline solutions. 



The cuticle may be isolated, from the surface of a 

 leaf for instance, by boiling for a few minutes in hydro- 

 chloric acid, and then washing with water. 



d. Callus. To be found on the plates of the sieve- 

 tubes. 



i. Soluble in sulphuric acid. 



ii. Stained by Hoffmann's blue, and by hsematoxylin. 

 The most delicate reagent for callus is the following : 

 to a quantity of chlor. zinc, iod., add an equal volume of 

 the ordinary solution of iodine in potassium iodide ; to 



