WOUND-CORK FORMATION IN THE SWEET POTATO 
By J. L. Weimer, Assistant Pathologist, and L. L. Harter, Pathologist, Office of 
Cotton, Truck and. Forage Crop Disease Investigations, Bureau of Plant Industry, 
United States Department of Agriculture 
INTRODUCTION 
The investigations reported in this paper were designed primarily to 
determine to what extent and under what conditions wound-cork is formed 
by the sweet potato and whether or not its formation serves in any way 
as a barrier to infection by Rhizopus trilici Saito. 
PERIDERM FORMATION 
The sections made from samples taken at the beginning of the experi¬ 
ments showed the condition of the tissue before any reaction to the 
wound stimulus occurred. The sections are made up of thin-walled 
cells varying somewhat in size but in the main rather large, with here 
and there a vascular bundle. The cells, with the exception of those 
comprising the vascular tissue, are more or less uniformly filled with 
starch. After 24 hours’ exposure in a moist chamber held at room tem¬ 
perature (about 25 0 C.) a distinct change is already noticeable. A layer 
of cells running parallel to the exposed surface, varying from 3 to 10 
deep, contains more starch than do those immediately beneath. No 
other change can be detected at this time, but on the second day the 
starch in the lower layer of cells becomes conspicuously less. About this 
time one or two indistinct cross walls are laid down in some of the cells 
in the starch-free area, so that the position to be occupied by the wound 
cork can be traced entirely across the section. In about 3 days the cork 
layer is pronounced, being from 2 to 4 cells deep, and the cell walls are 
considerably thickened. These changes are still more pronounced from 
the fourth to the sixth days. By the sixth day there is a continuous 
layer of cork, varying from 2 to 6 cells in thickness and lying from 2 to 
12 cells beneath the wounded surface. Between the cork periderm and 
the layer comprising the surface cells is a third layer from 2 to 4 cells 
in depth which contains little or no starch and whose walls are consid¬ 
erably thickened and suberized, as shown by the fact that they stain 
deeply with iodin green. The thickening of the cell walls becomes evi¬ 
dent about the third day. The cells immediately below the cork meristem 
merge into the cells containing the normal amount of starch. 
It was found by the writers that the rate of cork development is 
retarded at the bottom of wounds made by striking sweet potatoes 
against the sharp edge of a wire basket. Coutant (2) 1 also found that the 
1 Reference is made by number (italic) to “literature cited," p. 647. 
Journal, of Agricultural Research, 
Washington, D. C. 
yu 
Vol. XXI, No. 9 
Aug. 1, 1921 
Key No. G-243 
(637) 
