360 CouTANT: WOOD PERIDERM IN CERTAIN CACTI 
of them also give the suberin test, leaving, however, the large 
number of discolored cells unlignified, or giving the lignin test and 
no other. This phenomenon of cell walls giving the tests for 
both suberin and lignin Kiister (1, p. 165) finds in the callus 
tissue of poplar cuttings. He says he does not doubt that the 
same substance, or a similar one, is the cause of these reactions. 
However, this statement is perhaps open to question. There 
seems no reason why both suberin and lignin might not be present 
together. 
In wounds six and seven days old the phellogen has produced 
on the outside more suberized-walled phellem cells and on the 
inside what may be called a distinct phelloderm, the walls of which 
are pure cellulose (PLATE 9, FIG. 3). For the sake of convenience 
these tissues will be spoken of as phellem, phellogen and phello- 
derm, with the realization that, although analogous to the normal 
periderm of plants, they are not necessarily homologous. Although 
the phellem extends up through the hypoderm, few cells have 
been produced there. At this stage, still more of the cells in the 
discolored tissue have become lignified. 
A wound ten days old shows several marked advances. The 
periderm is in all about eighteen cells deep. However, the 
phellogen, instead of producing only suberized phellem tissue, 
has given rise to a few thick-walled lignified cells, five or six in a 
cross section, appearing on one side, about half way down the 
“V."" These cells were previously described in discussing the 
normal periderm. A further lignification of the discolored tissue 
is apparent. Another change is in the distribution of the stored 
starch. With the formation of waterproofing cells covering the 
wound, the normal activities of the tissues seem to be resumed, 
and small grains of storage starch appear in the phelloderm cells, 
and in the cortical cells below the phelloderm. These grains are, 
however, very few and very scattered. 
The next wound studied in detail was twenty-four days old. 
Considering the various zones of tissues which extend from the 
wounded surface into the interior of the plant, we have (1) on 
the outside, the cells, in no way discolored, which retain the original 
stored starch they contained at the time of wounding. Some 
enclose oxalate crystals which have increased considerably in size. 
