Suberin and Cutin 
25 
Most parasitic fungi contain cytases, or cellulose dissolving 
enzymes, which are excreted at the growing tips and could probably 
disintegrate a membrane with a cellulose basis, as for instance 
many wood-destroying fungi disintegrate lignified membranes 
without digesting the lignin. But unless a fungus hypha penetrates 
through stoma or lenticel, it finds itself met at the surface of the plant 
by an unbroken lamella containing no cellulose fabric at all and which 
is chemically so composed that it would seem unlikely that ordinary 
hydrolysing enzyme action would decompose it. 
It is therefore interesting to note that Blackman and Welsford(i) 
in the case of the parasitic fungus Botrytis, describe it as appearing 
to penetrate the cuticle by forcing its pointed tip forward under 
hydrostatic pressure, so that it wins its way through by mechanical 
displacement of the actual waxy substance of the cuticle. 
The absence of cellulose in the external layers of the plant is a 
conception that does not seem to find favour with English writers, 
but possibly too much weight has been attached to the criticism of 
Cross and Bevan( 3 ) ( loc . cit. p. 228). The views expressed in this 
English monograph, where cork is described as an “ adipo-cellulose ” 
or fatty substance allied to cellulose, are quoted in all English bio¬ 
chemical texts. Cross and Bevan briefly state van Wisselingh’s 
point of view (although they quote him as “van Wissenburgh”) 
and then brush it aside because, after removal of the suberin from 
cork by a special bisulphite process used on a macro-chemical scale, 
cellulose is indisputably present in the residue. But this is entirely 
beside the point. Reference to van Wisselingh’s papers will show 
his frequent description of cellulose layers within cork cells, internal 
to the suberin lamella. The only point at issue is whether cellulose 
also occurs within the suberin lamella itself. 
Van Wisselingh gives reasons, that seem very adequate, for con¬ 
cluding that it does not, and this being the case it will follow, 
unless the middle lamella has a cellulose basis, that the cork will 
provide an unbroken cellulose-free layer just as the cuticle does. 
Suberin and Cutin not Chemical Entities but Aggregates 
of Substances 
Van Wisselingh concludes that suberin is a mixture of substances 
because in his observations he sees these substances melting out of 
the lamella at different temperatures. In general he recognises two 
series of substances as present; the one series, melting at relatively 
