254 J- H. Priestley and L. M. Woffenden 
along walls and intercellular spaces, in an unbroken layer just below 
the cut surface. Appel emphasises the fact that this impermeable 
deposit is of great practical significance, because at an average 
temperature it forms within twelve hours, whilst the periderm 
formation, the product of a meristem, will only arise within 48 hours 
and will not be a very effective barrier until somewhat later. The 
pathogenic organism Bacillus phytophthorus (Appel) can penetrate 
deeply into a cut surface within a few hours: as, however, it fails to 
penetrate if sown upon the cut surface after twelve hours, it is clear 
that its entry is prevented by this preliminary brown deposit. 
This brown deposit is considered by Appel to be suberin and he 
confirms the statements of earlier investigators, Kny(i 4 ) and 
01ufsen(i8), that the formation of suberin is definitely connected with 
access of oxygen. Thus he explains its inadequate formation in 
potato tubers placed after cutting in warm dry air, in that the cut 
surface then dries rapidly and the entry of air is prevented by the 
hard dry surface. Suberisation in such cases therefore occurs at 
irregular depths in small patches around intercellular spaces and not 
in a continuous layer. As the hard outer layer often cracks irregularly 
and in any case softens and permits the entry of fungi and bacteria 
when the tuber is placed in moist earth, the potato sets are very 
subject to disease when planted. The grafting experiments of 
Kabus(i 3 ) when cut tubers of potato were pressed closely together 
gave further evidence that the preliminary suberisation was con¬ 
nected with air, as if these cut surfaces made good contact, subsequent 
staining with Sudan III showed patches of suberin developed only 
at places where the two surfaces had imprisoned an air bubble. 
The access of air to the cut surface may be prevented in various 
ways; in Kabus’ experiments the potatoes were cut under previously 
boiled water, the cut surfaces pressed together and then bound 
together by some material to prevent subsequent access of air. 
Under these conditions, even potatoes from which all buds had been 
removed would slowly develop cellular outgrowths, of the nature 
of intumescences, which interpenetrated the two opposed surfaces 
and finally bound them together, without hindrance from suberisa¬ 
tion and without the intervention of a meristem. Similarly both 
01 ufsen(i 8 ) and Kabus found that if a piece of potato were removed 
by a cork borer, then pressed back into the hole and the ring-shaped 
cut in the outer surface sealed, no suberisation occurred at the cut 
surface, but the spaces created by the injury were bridged by pro¬ 
liferating cells without the intervention of a meristem. Kabus cut 
