362 COUTANT: WOOD PERIDERM IN CERTAIN CACTI 
which are (4) the thick-walled lignin cells, usually two, but 
occasionally three deep. Inside is (5) more suberized tissue, which 
arises directly from (6) the phellogen. The phelloderm (7) below 
this, averages from ten to twelve cells deep, but is considerably 
thicker near the base of the “‘V,’”’ where the appearance of some, 
of the cells seems to indicate the formation of a second meristem. 
The walls of the cells in this region are of cellulose, and the con- 
tents include small, probably newly formed oxalate crystals, and 
a considerable quantity of stored starch. The cortical cells (8) 
underneath the phelloderm have now practically regained their 
normal starch content. 
TExtT Fic. 3 will aid in the study of a thirty-one-day-old 
wound. The periderm is more developed, and now includes in 
certain areas two more alternating layers of thick- and thin-walled 
tissue. However, the big advance made here comes in the further 
development of the second layer of meristematic tissue forming 
within the old periderm, parallel to the exposed surface. The 
function of this meristem seems to be to produce new vascular 
bundles. 
These bundles develop first near the innermost portion of 
the wound, where the meristem was first seen to form. Kiister 
(1, p. 164), in his discussion on the histology of callus, mentions the 
formation of tracheids, especially in the inner layer. By the 
union of many of these tracheids, he says, primitive vascular 
bundles, and a wood-like tissue are produced. However, here in 
the cacti, we have not just scattered rudiments of bundles, but 
distinct masses arranged in a definite position. 
Eventually the first wound-meristem extends peripherally into 
the hypodermal tissue, but not until it is elsewhere very well 
developed. Another point to be noticed is that where the bundles 
were cut through, or where they are merely in a line with the 
wound phellogen, their prosenchymatic as well as their parenchy- 
matic cells seem capable of reverting to a meristematic condition. 
Whether the stereome and hadrome tissues are able to revert is 
possibly questionable, but in these wounds it does not look as if 
the meristematic cells adjoining them had arisen from parenchyma 
cells some distance away. 
