Suberin and Cutin 
19 
Von Hohnel attached most importance to the reaction of suberin 
with oxidising agents such as nitric acid or Schulze’s macerating 
solution (potassium chlorate in nitric acid), a reaction which he 
describes as the cerin or cerinic acid reaction because the suberin 
is completely oxidised to a waxy substance which he regards as an 
oxidation product of suberin and terms cerinic acid; lignin gives no 
such oxidation product. 
For micro-chemical purposes the sections are placed in Schulze’s 
macerating solution and warmed gently under the coverglass, the 
lignified walls are gradually destroyed, the walls becoming more and 
more transparent and behaving like pure cellulose. Suberised (or 
cutinised) walls are much more resistant. They remain opaque and 
dark in the cold reagent and sharply contrasted therefore with all 
other walls; on warming an ebullition of gas is seen from the cork 
walls, but they swell very little until at a stage in the process of 
warming they collapse, the outlines becoming very wavy and swollen, 
the contents of the membrane largely melting up into oily masses 
which finally become spherical and globular and solidify on cooling. 
These globules are said to consist of cerinic acid, a substance first ob¬ 
tained by a similar process by Doepping(5). Doepping gave it the 
name, under the impression it had arisen by the oxidation of cerin, a 
substance found to a small extent in cork. Von Hohnel retains the 
name whilst giving very sound ideas for regarding it as an oxidation 
product of suberin. 
Another reaction of great importance in the identification of 
suberin, as was also emphasised by von Hohnel, is its behaviour 
with caustic alkali. Both lignin and suberin dissolve in concentrated 
caustic potash on warming, but von Hohnel points out that the 
suberin containing wall is usually thrown into more marked dis¬ 
organisation by the removal of the suberin, which gradually dis¬ 
appears from the wall on heating, usually going into solution with 
a strong yellow colour. The lignin dissolves out in a very different 
way, leaving the wall little altered by its departure; in the solution of 
lignin a marked yellow coloration is often observed so that too much 
importance cannot be attached to this colour change. 
Anticipating later work this distinction may be emphasised. In 
the lignified membrane lignin and cellulose seem to be present in an 
intimate physical mixture 1 , and after solution of the lignin the out¬ 
line of the wall remains practically unchanged. But in the cork wall 
1 The reader is referred for recent evidence on this point to the very- 
interesting paper by Robinson (13). 
