358 CoOuUTANT: WooD PERIDERM IN CERTAIN CACTI 
developed, possibly due to lack of available oxygen or perhaps to 
the lessening of the transpiration. See TExT FIG. 1. The latter 
is suggested by a fact mentioned by Kiister (1, p. 187), that a 
preliminary condition for the production of wound-cork is that 
at least a small degree of transpiration must be possible for 
the exposed tissue. Here, as pointed out, the cells on the cut 
surface near the periphery must have at least a normal amount 
of transpiration, while those deep in the cut can have much less. 
Fic. Cross section of a four-day-old wound of an Opuntia discata plant, 
showing os characteristic V-shaped cut and the formation of the wound-phellogen 
only in the outer parts of the wound, X 28. a, phellogen layer. 
Another change from the three-day-old wound is that the walls 
of the discolored cells nearest to and outside of the developing 
phellogen, have become lignified. 
A study of a five-day-old wound can best be made by referring 
to Text FIG. 2. The phellogen is clearly distinguishable and has 
produced, except near the periphery, what may be spoken of 
as a distinct phellem. This phellem is about three cells deep, 
and the walls are highly suberized. The lignified cells just outside 
