IgOI | SOUTHWESTERN CACTACEAE 47 
more bundles are embraced by a single sheath, and this process 
apparently continues until all the bundles of the rib are thus 
bound together. This sheath is not the ordinary one of com- 
pressed sclerenchyma so commonly found next the sieve tubes. 
This outer one seems to be a special structure, composed of the 
obliquely pitted fiber tracheid so often mentioned, which here 
inclines more than ordinarily to the tracheid form, with wide 
lumen and with the extremities of the cells scarcely pointed. 
The individual cells are much larger than those of the same 
nature found in the xylem. The ribs alternate with the ridges 
of tubercles, botii undergoing more or less branching. A full- 
grown rib, according to measurements taken from dead skeletons, 
seldom exceeds 3™ in thickness. The phloem has the usual 
development of sieve tubes; the xylem is made up almost 
wholly of scalariform tracheids, an element so far noted only 
in the root. Besides these, there also occur fiber tracheids, 
undoubtedly in increasing proportion as the size of the rib and 
the corresponding function as a support increases. 
The absorptive root differs from the others described in 
having in the phloem an abundance of fiber tracheids, forming a 
wedge pointing outward, starting just outside the sieve tubes of 
each bundle. These cells are also numerous in the xylem, 
together with the scalariform tracheids. Crystals are common 
in the cortex as well as in that of the stem, and are large and 
closely aggregate. They do not seem to be entirely homoge- 
neous, but take some foreign substance asa nucleus. At times 
they appear to grow by concretion, showing when viewed in 
optical section a series of concentric circles. 
The necessity for a fibrous strengthening in the absorptive 
root of this large form can best be understood by examining the 
method of growth. In starting as a seedling, the plant begins 
life with a base slightly sunken in the ground, but far narrower 
than the heavy column which it is to support. Therefore, as 
this column develops, the whole becomes very top-heavy, and 
would certainly fall were slightly greater surface exposed to 
the wind. The anchoring root, at this time well developed, holds 
