CouTANT: Woop PERIDERM IN CERTAIN CACTI SY f 
College greenhouse, but, although giving fairly good results, they 
were by no means as dependable as those obtained from material 
wounded and killed directly in its natural habitat in Tucson. 
Material of this character, collected by Dr. H. M. Richards in 
1911 and by Dr. D. T. MacDougal in 1914, serves as the basis 
for the observations which follow. : 
Practically all of the studies have been made from transverse 
sections through the stems, where, owing to the above mentioned 
opening of the wound, the two exposed surfaces form a “V.” 
Hand sections stained with Bismarck brown served to show clearly 
all cell walls, and the unstained portions were easily tested for 
cutin, cellulose, lignin, etc. Although most of the figures were 
made from such hand sections, those cut with a microtome were 
used for reference. 
he only visible change during the first day after wounding 
is the partial loss of starch in a region running parallel with the 
exposed surface of the wound, but not bordering directly upon it. 
By the second day, that region is entirely starchless, and the 
reaction with iodine of the starch in the cells on the wound surface 
does not give the normal purplish blue, but more of a reddish 
color, suggesting the presence of a greater proportion of erythro- 
dextrin. 
In three-day-old wounds, some of the oxalate crystals in the 
most exposed cells have increased in size. The starchless area in 
the upper part of the “V”’ has become divided into two practically 
equal parts, by the foreshadowing of a meristematic layer running 
parallel to the surface of the wound. The cells in the outer region, 
that is, between this future meristem and the cut surface, are some- 
what discolored. The cells comprising this prospective wound- 
phellogen are one layer in depth, and although no divisions have 
as yet taken place, there seems to be an adjustment of the contain- 
ing cytoplasm to form a flat plate, continuous from one cell to 
another, and parallel to the wound surface. PLATE 9, FIG. 2, will 
give an idea of how this appears in section. 
A wound four days old shows the meristematic or phellogen 
layer more clearly, especially near the periphery, where the cells 
seem about to divide. Near the base of the cut, particularly if 
the wound is deep and the circulation of air slight, it is un- 
