356 § COUTANT: WOOD PERIDERM IN CERTAIN CACTI 
A single layer of this lignified tissue is never deeper than three cells, 
arranged one on top of another; the outer tangential and the 
radial walls are thicker than the inner tangential ones, and in 
consequence the peripheral portions of the lumina are dome- 
shaped, as is so common in thickened cutinized tissue of the 
epidermis. In very thin sections stained with Bismarck brown, 
distinct striations can be seen in the walls, coarser near the lumen, 
and becoming finer toward the exterior; while running at right 
angles to these striations are numerous fine pores. PLATE 9, 
FIG. 5, gives an optical transverse view of this tissue. The middle 
lamellae are shown, but the striations and pits above referred to 
are not indicated. In describing the origin of this particular 
tissue, Schleiden (3, p. 352) says that there arises a thick, yellow- 
brown, granular, slimy mass, in which eventually cells are formed. 
At present such an hypothesis is not to be accepted. However, 
although omitting any reference to lignification, he brings out 
clearly the fact that in the mature periderm there are alternating 
zones of suberized thin- and thick-walled tissue. 
EFFECT OF WOUNDING ON NORMAL TISSUE 
Having studied the normal tissues, with especial reference to 
the periderm, the next problem was to find out what reactions 
would follow as a result of injury. Wounds were brought about 
by making longitudinal slits down the plants with a razor, usually 
cutting deeper than the bundle ring, but never beyond the center 
of the pith. A contraction of the cells near the cut surface 
resulted, thereby opening up the wound, and exposing the tissue 
to the air. After the elapse of definite periods this material was 
placed in alcohol. Some stems were wounded with glass, in 
order to avoid a possible stimulus which might result from the 
acids of the tissues acting on the steel of the razor, and the resultant 
formation of soluble iron salts. In running a parallel series, how- 
ever, there was found to be no difference in the rate of wound 
reaction between the two, so the use of the razor was adhered to. 
All of the material used came originally from Tucson, Arizona. 
In the spring of 1913, cuttings were sent from Tucson, which were 
immediately wounded and killed, but gave unsatisfatory results. 
Plants from Tucson were wounded while growing in the Barnard 
