Igor] SOUTHWESTERN CACTACEAE 45 
close contact at an early stage. The phloem portions, however, 
remain distinct for each bundle. By this slight divergence, 
considerable medullary connection is left between the pith and 
the thick layer of storage tissue outside the poorly developed 
bundle cylinder, a far greater connection than could occur with 
the usual distribution, thickening, and branching of bundles seen 
in dicotyledonous stems. 
This peculiar grouping will be noted 
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Fic. 6.—Diagram of position of bundles in Echinocactus Wislizeni and others. 
in a far greater degree of perfection in Cereus giganteus, next to 
be described. Here, as there, there seems to be no anastomosis 
of these groups of bundles, the incipient wooden ribs. In 
Opuntia, on the contrary, where this anastomosis of bundles 
likewise occurs to some degree, a still further reticulation is 
found, in that the groups as units anastomose. 
As regards more minute structure in £. Wisiizent, the phloem 
is composed mainly of sieve vessels, the outer of which, together 
with the adjacent parenchyma, are composed and rendered 
sclerenchymatous, forming a sheath, here separate for each indi- 
vidual bundle. The xylem, at least in later portions, is com- 
posed entirely of spiraled tracheids of uniform size, but not in 
definite rows, the spiral also being of the ordinary type. A 
