1901 | SOUTHWESTERN CACTACEAE BL 
xylem shows an occasional ringed duct, apparently an early ele- 
ment, but is principally composed of scalariform and fiber 
tracheids. Starch is present in considerable quantity through 
the stem. 
The cortical parenchyma of the absorptive root is thickened 
in many places for storage pur- 
poses. The cells are filled with 
a thick mucilage, which aids in 
holding the water. A consider- 
able deposit of starch, as well ip 
: us — AIF AL I 
as of the regular calcium oxa C Oger 
late crystals, occurs in the me- ss LOAF S 
F ah “A SBLBO, S28 
dullary rays, but not at all in (ae eeope 
the cortex, an apparent differ- 
entiation of regions for vari- 
ous reserve substances. The 
phloem is of ordinary charac- 
ter, with a slightly developed Fic. 
sheath ; the xylem is composed °! Omntia arbuscula. 
mainly of scalariform tracheids of various sizes, but contains a 
few fiber tracheids as well. A rather peculiar phenomenon is 
presented by the convergence of the cortical tissue toward the 
center of the phloem. Whether this was to hasten transfer of 
materials between cortex and phloem in young stages, before 
the sheath was thickened, in a way analogous to the clustering 
of palisade about the leaf bundles, which so commonly occurs, 
is a question which at once suggests itself, but upon which I am 
able to throw no light at present (fig. 9). 
OpunTIA LEPTOcAULIs DC.—The epidermis contains crystals 
in cells specially enlarged to receive them. The hypoderma is 
of four or five layers, and the assimilative tissue is elongated 
very slightly. The parenchyma of pith and cortex is nearly 
colorless, thin-walled, and contains considerable starch, evidently 
of the translocation variety. This species is not so succulent as 
the majority, and has an earlier development of the bundles. A 
point in which it differs from all the rest examined is in its 
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G. 9.—Portion of b 
