52 BOTANICAL GAZETTE [JULY 
possession of a well-marked interfascicular cambium in the stem, 
allowing the medullary tissue to increase in proportion to the 
growth of the bundle, instead of having the bundle compress and 
supersede it, as is apparently the condition in the others. That 
even here this cambium is not so active as that of the bundle seems 
to be demonstrated by the fact that outside the phloem the scleren- 
chyma sheath still appears compressed, though to a less degree. 
The xylem is composed mainly of fiber tracheids, intermingled 
with which are scattering spiraled ducts. The ordinary Opuntia 
reticulation occurs together with the finer anastomosis of the 
bundles of a single strand. Between these closely connected 
unbranched bundles the medullary rays develop very thick walls, 
apparently lignified, and are so filled with a deposit of starch 
that in section they appear much more opaque than does the 
rather close-grained wood on each side of them. Mucilage 
occurs, but not so abundantly as in the majority of cases, and 
crystals of rather small size are scattered through all the paren- 
chyma tissue. 
The root does not materially differ from those already 
described. The cortical parenchyma is slightly thickened, and 
the xylem is composed of the scalariform and fibrous types of 
tracheids. 
SUMMARY AND CONCLUSION.—Several inferences may be 
drawn from the foregoing descriptions. In the first place, it 
will be seen that the roots, as regards branching and elements 
concerned, apparently undergo but slight variation, being com- 
posed, in almost all cases,of fiber tracheids and scalariform 
tracheids in the xylem, and of sieve tubes only in the phloem. 
To this general rule, as regards xylem portions, Mamillaria Gra- 
hami is the only exception among the forms here compared, 
and even in this the tracheid appears in greater prominence than 
the true duct. This exception, however, is enough to suggest 
that a more comprehensive examination will reveal more of a 
similar nature, and also perhaps disclose some links in the evolu- 
tionary chain between this and and the ordinary type. 
In the stem, on the other hand, there is a great range of 
