42 BOTANICAL GAZETTE [JULY 
walls during the season. All'the broadening of the bundles, there- 
fore, occurs at the beginning of a period of growth. Likewise, 
when a branching is to occur, the resulting medullary ray appears 
at the beginning of the season, as shown in the drawing. 
It seems likely that the flattening of the spiral in the tra- 
cheids, which compose most of the xylem, is 
what gives the elasticity so prominent in this 
bundle. Such a flattening inwards would allow 
a far greater stretching of the spiral than would 
be possible in the commoner round type. A fact 
which strengthens this assumption is that the 
walls of the tracheids in a preparation will be 
; seen to be wrinkled between the spirals (jig. 5). 
ee aie living condition those were in all proba- 
tracheid of flattened 
spiral type from bility perfectly straight, and the turns of the 
xylem of stem bundle spiral at a greater distance apart. 
ee Another point which can be deduced safely 
from an examination of the figure is with respect to periodicity 
of growth. From the comparison with the amounts of earlier 
years, it will be seen that the xylem for the season was about 
half formed at the time of collection of material. As this took 
place in the latter part of December, it seems probable that 
the season of growth in the southwest is limited by the sum- 
mer drouth and intense heat, rather than by the winter cold. 
The branching of bundles and formation of secondary 
medullary rays in the stem is not of common occurrence in the 
Cactaceae, so far as my investigation goes. In most of the forms 
to be described a different state of affairs will be noted. The 
bundles of the root, however, branch in the ordinary manner in 
all the species I have examined. 
The absorptive root* of Cereus Fendleri has a cork four to 
six cell layers in thickness. Successive layers of this kind are 
formed, with parenchymatous intervals, so that the bark is being 
constantly shed in flaky fragments, and is with equal rapidity 
replaced. The cork cambium lies a short distance outside the 
4 Bor. GAZ. 30: 348. 1g00. 
