Igor] SOUTHWESTERN CACTACEAE 43 
secondary phloem of the bundles. This phloem is composed 
almost wholly of sieve tubes which, when examined January 1, 
had no callus visible. Lateral plates occur in abundance, in 
addition to those at the ends of the cells. 
The xylem has but two elements, at least*in the secondary 
portions. The more prominent, and by far the more abundant 
of these, is the tracheid, with ellipsoidal pits in more or less 
regular lines, an element which, from the shape of the pits, 
hardly coincides with the ordinary scalariform vessel, as seen in 
Pteris, for instance, but which, for convenience, I shall here 
designate by that term. Though the separate cells are compar- 
atively short, there is often a continuous passage through several 
(a transition stage toward the true trachea), the walls between 
the adjacent cells being partially dissolved, and represented by 
nothing more than athick and distinct ring. The other element 
is the obliquely pitted fiber tracheid, which apparently supplants 
the ordinary wood fiber in the majority of the Cactaceae.s The 
medullary tissue is but little compressed in the rays, giving a 
rather loose structure to the root as a whole. Crystals here, as 
also throughout the entire plant, are very few in comparison to 
their number in all the other species examined, occurring in the 
various parts of the parenchyma tissue, always in the aggregate 
form. 
The anchoring root differs from the absorptive chiefly in the 
relative numbers of fibrous and vascular elements, as stated ina 
previous paper,® in which the phrase ‘‘ wood cells’’ was used by 
mistake for “ fiber tracheids.”’ There is also a slight difference 
in the amount of sieve tissue developed in the phloem, the 
greater amount belonging naturally to the absorptive root. In 
Species where the two systems are not clearly differentiated, the 
elements are more nearly equal as regards amount of space 
Occupied by each. As a rule, the only difference between the 
two roots is one of proportion of elements, not of their variety. 
5Fora drawing of this element see Strasburger’s 7 vxt-book of Botany, translated 
by Porter, fg. 143 ft. 
° Bor. Gaz. 30: 348. 1900. 
