646 Journal of A gricultural Research voi. xxi, no. 9 
all decayed in 5 days. The fungus, however, was not able to soften the 
hardened layer of tissue over^ the healed surface. The inability of 
Rhizopus tritici to break down this protective wound covering was 
demonstrated further by placing sections of this tissue in sweet-potato 
decoction upon which this fungus had grown for 3 days. Freshly cut 
sweet-potato blocks used as controls were entirely softened in less than 
48 hours, whereas the tissue over the wound was not affected after 
4 days. 
The length of time necessary to form an impenetrable barrier over a 
wounded surface no doubt varies with the conditions. Data presented 
in Table III show that under the conditions realized in some experiments 
a considerable retarding effect was exhibited after 48 hours, while in 
other cases there seemed to be no delay or inhibition of decay after 
healing for 6 days. 
It has been demonstrated (r) that a cut surface of the Irish potato 
healed to such an extent in 12 hours that a virulent culture of Bacillus 
phytophthorus Appel was unable to penetrate the tissue; furthermore (5), 
that wounded potatoes when inoculated 3 hours after being placed in an 
incubator at 30° C. failed to become infected, a protective covering 
having already formed. 
It has also been found (4) that the wound cork is very effective in 
preventing infection of the sweet potato by Mucor stolonijer Ehrb. 
Similar results were obtained by other workers with these and other 
crops. 
The fact that the presence of a definite cork layer could not be demon¬ 
strated in some cases where decay was inhibited or retarded would seem 
to indicate that this layer is not alone responsible for the protection of 
the wounds. Certainly no cork layer could have been laid down in the 
instances mentioned above, where infection of the Irish potato was in¬ 
hibited by 12 hours’ healing and in the case where it failed to become 
infected through a surface healed for only 3 hours. No doubt the suberi- 
zation of the surface cell walls is a factor in preventing the entrance of 
microorganisms, and even the drying out of the injured cells may exert 
some retarding or inhibiting action against decay-producing organisms. 
SUMMARY 
(1) Sweet potatoes have the ability to form a cork layer over wounds 
when the environmental conditions are favorable. 
(2) The production of a cork layer is preceded by the formation of a 
layer of starch-free cells, usually 3 to 10 cells deep, beneath the injured 
surface. Cross walls begin to appear from the second to the third day, 
and by the fourth to the sixth day a distinct layer of cork cells forms a 
covering over the wound. 
(3) Temperature and humidity are important factors in regulating 
the formation of cork in the sweet potato. This process took place more 
