1901 | SOUTHWESTERN CACTACEAE es: 
structural deviations, which seem to take place along definite 
lines and by definite steps. This variation extends to bundle 
branching and further reticulation, to the extent of succulence, 
character of parenchyma of pith and cortex, development of 
mucilage, and even to the kinds of elements composing the 
xylem of the bundle. In the phloem alone there is little 
change. The practically universal substitution of fiber tracheids 
for wood fibers, and the equally common appearance of tracheid 
elements in place of true ducts, except in early growth, points 
certainly to a general regression. The fiber tracheid, moreover, 
is variable in itself. In most cases it is quite sharply pointed and 
thick-walled, with a lumen, though broader than that of an 
ordinary wood fiber, still rather narrow. In a few instances, 
however, this same element loses its pointed character, widens 
its lumen still more, and except for its thick walls and oblique 
pits, scattered and exceedingly small, closely resembles an 
ordinary tracheid. This is very well shown in Cereus giganteus, 
a rather anomalous example, however, since here the fiber 
tracheid supplants a bast fiber instead of a wood fiber, and does 
not undergo an equal enlargement in the xylem portions of the 
bundle. 
Crystal distribution and various storage peculiarities should 
be looked upon more as specific differences, I believe; likewise 
the thickness of hypoderma. 
A word or two may be said of Opuntia leptocaulis, The early 
development of the bundles, the smaller degree of succulence, 
the persistent character of the leaves, at least in some varieties, 
as noted by Coulter,’ the very slight elongation of the assimi- 
lative tissue, and above all the persistence of the interfascicular 
cambium, all tend to place this form in structure much nearer 
the ordinary dicotyledonous stem than any of the others exam- 
ined. It seems safe to conclude that this species comes lower 
in the evolutionary line of adaptive modification than the rest 
of the Opuntias here mentioned, and may be considered nearer 
the primitive form. 
7Contrib. U. S. Nat. Herb. 3: 456. 
