28 
J. H. Priestley 
special properties of periderm, impermeability to water and resist¬ 
ance to sulphuric acid, are supposed to be due. 
2. Cutin is a substance present as a continuous external lamella 
on the outer wall of the epidermis in leaf and stem to which is 
assigned the same rdle in reference to the cuticle. 
3. The properties of these suberin and cutin layers may be 
jointly defined as insolubility in and impermeability to water, con¬ 
siderable insolubility in fatty solvents, great resistance to concen¬ 
trated sulphuric acid, ready oxidation by nitric or chromic acids 
and ready solubility in warm alkali; they are stained by fat stains 
such as Sudan III or scarlet red. 
4. In the layers in which suberin or cutin are present in the 
plant wall no cellulose can be detected; the importance of this 
conclusion to students of plant pathology is emphasised. 
5. On chemical grounds suberin may be regarded as an aggregate 
of variously modified forms (condensation products or anhydrides) 
of certain organic acids, the suberogenic acids. The chemical con¬ 
stitution of these acids requires further elucidation before the problem 
of the origin of the suberogenic acids can be satisfactorily attacked. 
To a small extent the suberogenic acids may be present in suberin 
in combination with glycerine as substances of the nature of true fats. 
6. These suberogenic acids, some of which have been obtained 
crystalline and in the pure state, are usually soluble in fatty solvents 
at any rate on warming, but the anhydrides or condensation pro¬ 
ducts formed from them are completely insoluble in fatty solvents 
and only give rise to the original suberogenic acids on saponification 
with alkali. 
7. One of these suberogenic acids, phellonic acid, gives colour 
reactions with iodine reagents which are responsible for the erroneous 
impression that cellulose is present in the suberin lamella. 
8. There is not the same experimental evidence in the case of 
cutin, but there is every reason for thinking that this is a similar 
aggregate of modified forms of “ cutinogenic” acids. 
9. Differences between suberin and cutin of different plants, or 
between the suberin and cutin of the same plant may be traced 
(1) to different organic acids and different proportion of those acids 
entering into the composition of the aggregate, (2) to the different 
conditions under which these acids have been transformed into the 
modified form in which they are present in the mature aggregate. 
Botanical Department, 
The University, Leeds. 
