638 
Journal of Agricultural Research 
Vol. XXI, No. 9 
cork meristem in the base of a V-shaped cut was undeveloped in the 
4-day-old sections. This is thought to be due to a lack of available 
oxygen or to the lessening of transpiration. This latter explanation is 
suggested by Kiister (7), who showed that at least a small amount of 
transpiration is necessary for wound-cork formation. 
EXPERIMENTAL METHODS 
The sweet potatoes used in these experiments were of the Yellow 
Jersey variety, grown on the Potomac flats near Arlington, Va. The 
first samples from which the sections were cut for the determination of 
wound-cork formation were taken from potatoes while still in the ground. 
After the potatoes were put in storage samples were taken and studied 
at different times throughout the storage period. For the most part the 
potatoes were halved and blocks of tissue about 0.5 by 0.5 by 2 cm. were 
removed from the cut surface in such a manner that sections made across 
the short diameter would be perpendicular to this surface. A control 
sample was taken when the experiment was started, and others were 
taken at stated intervals of time thereafter. The samples were dropped 
at once into a fixer made by adding 2 cc. of commercial formalin to 100 
cc. of 70 per cent alcohol. After fixing for 24 hours or longer the speci¬ 
mens were dehydrated with alcohol and embedded in paraffin. Sections 
were cut for the most part 10 n in thickness, stained with a weak solution 
of iodin green in 70 per cent alcohol, ammoniacal gentian violet, or in 
Pianeze Illb stain, and mounted in balsam. 
In order to determine whether wound cork is produced as readily at 
the center of the sweet potato as it is near the vascular ring, samples 
were taken from both regions. A study of sections from these regions 
shows that although a normal cork layer is formed in both cases, it is 
produced more quickly in the vicinity of the vascular ring. In the latter 
case the cork layer laid down in two days is to all appearances equal to 
that formed at the center of the potato in three days. A somewhat 
different condition was found in the Irish potato (9, 12), where periderm 
formation in the seed pieces is usually not so well developed in the central 
as in the peripheral portion. 
CONDITIONS AFFECTING CORK FORMATION 
Factors influencing wound-cork formation have been considered by 
numerous writers, who agree that a certain amount of moisture is neces¬ 
sary. When the cut surfaces are allowed to dry too rapidly, a desiccation 
and death of the surface cells soon takes place. This results in the forma¬ 
tion of a hard surface layer beneath which cork periderm seldom forms. 
On the other hand, when the escape of water from the wounded surface 
is retarded, a cork layer usually develops beneath the injured surface. 
The degree of humidity present in the ordinary moist chamber is such 
that cork is produced by the sweet potato. Halves of sweet potatoes 
