SPECIAL METHODS. 15 r 



Iodide of zinc, but always yellow or brown ; and are in- 

 soluble in concentrated sulphuric acid. 



But, according to Fr. von Hohnel's researches (I), the fol- 

 lowing reactions are especially characteristic : 



Concentrated caustic potash solution causes in the cold a 

 yellow coloring of suberized membranes, which increases in 

 intensity when they are warmed in this fluid. The suberized 

 membranes take, at the same time, a lineate or granular 

 structure which becomes plainer on further warming. OR 

 boiling in the same solution, the large yellow drops which 

 are formed often escape entirely from the membrane. 



Schulzes macerating mixture (cf. 9, i) is resisted longest 

 by suberized walls, of all the modifications of cellulose; but 

 they finally run together, on long boiling in the fluid, into- 

 oil-like drops whose substance is termed eerie acid and is sol- 

 uble in hot alcohol, ether, chloroform, benzol, and a dilute 

 solution of caustic potash, but insoluble in carbon bisul- 

 phide. 



Concentrated chromic acid either does not dissolve the 

 suberized membranes at all, or only after acting for a day ; 

 while all the other modifications of cellulose, except fungus- 

 cellulose, are dissolved by this acid in a short time. 



267. For distinguishing between suberized and lignified 

 cell-membranes, chlorophyll and alcannin may be used. 



Correns (II, 658, note) first recognized the fact that chlo- 

 rophyll stains the cuticle and suberized membranes deep 

 green, while the lignified and pure cellulose walls are not 

 colored. For this purpose, a freshly prepared alcoholic 

 solution of chlorophyll, as concentrated as possible, should 

 be allowed to act on the sections in darkness for a quar- 

 ter of an hour- or longer. The sections may then be ex- 

 amined in water. They cannot be preserved by ordinary 

 methods. 



For staining with alcannin, a solution of this substance in 

 50$ alcohol is used, in which the sections are left for several 

 hours or longer. All suberized membranes and the cuticle 

 take a red color which is not so deep as with any fats which 

 may be present, but is always clearly visible. 



