CouTANT: WooD PERIDERM IN CERTAIN CACTI 355 
angles to the periphery. The outer five or six rows are normally 
rich in chloroplasts, and practically all of the cortical tissues con- 
tain a plentiful supply of stored starch. In seedlings a single- 
layered epidermis covers the distinctly parenchymatic cortical 
cells. However, as the plant matures, the outer cortex gives rise 
to a clearly differentiated tissue, the hypoderm, socalled by 
Solereder in his Systematic Anatomy of the Dicotyledons (4). 
The cells of the hypoderm are, in both opuntias here studied, from 
five to seven layers in depth, and decidedly collenchymatous in 
character. Schleiden (3, p. 348) describes this condition as arising 
through loss of water, from cells originally mucilaginous. PLATE 
9, FIG. 1 will serve to give an idea of the thick cellulose walls, 
traversed by pit canals which radiate out from the comparatively 
small lumina, indicating protoplasmic continuity between the cell 
contents. The outer layer, which lies just underneath the epi- 
dermis, is characteristically so filled with oxalate crystals as to 
almost obliterate the walls. Occasional crystals are found in the 
other cells. This tissue has been described for different species 
of Opuntia by Lauterbach (2, pp. 259-264), as well as by the two 
authors mentioned above. 
The epidermis is a single layer of cells in thickness and covered 
thickly with cutin, as shown by staining with Sudan III and heat- 
ing. The stomatal openings extend through the hypodermal 
tissue down to the cortical cells. The guard cells are somewhat 
sunken, and the whole cavity is lined with cutin continuous with 
that on the outside. This fact was brought out by Mohl (see 
Solereder, 4, p. 408) in his work on the cacti. : 
The normal periderm, mentioned in the introduction as forming 
in plants several years old, arises superficially, and gives the ap- 
pearance of circular markings on the surface. When examined in 
a transverse section of the stem, it is seen to be composed of 
alternating zones of thin- and thick-walled tissue, each zone being 
itself several layers of cells in thickness (PLATE 9, FIG. 6). The 
thin-walled cells appear brick-shaped, and superficially greatly 
resemble meristematic tissue, except that their walls are highly 
suberized. The thick-walled cells are, on the contrary, lignified, 
reacting with phloroglucin and hydrochloric acid as did the 
stereome mentioned in discussing the structure of the bundles. 
