420 Hill and Freeman . — The Root-Structure 
wards; the tissue external to and opposite the phloem-groups 
is, however, much thicker- walled. 
Finally the cortex withers, and thus are formed roots of 
a high degree of hardness and bearing spines, the structure 
of which will be considered below. 
For the sake of comparison the smaller and less modified 
roots, the essential function of which is absorption, were 
examined. These organs do not depart in any important 
feature from those of the larger roots as set forth above. 
Those differences that do obtain are to be solely attributed 
to the relatively large size of the latter as already described ; 
thus instead of possessing thirty xylem-groups, the smaller 
roots never exhibited more than eight or ten. As regards 
histological features there are again no essential features of 
difference between the two forms, and in both the tissue 
directly bordering on the outer side of the phloem is the first 
to become lignified. 
Figs. 5 and 6 illustrate the structure of these smaller roots. 
Anatomy of the large spines. 
These thorns originate as very thick lateral roots (Fig. 7), 
and it is only at the extreme apex that a normal root-structure 
can be discerned. The phloem and xylem of the spines are 
connected respectively to the similar tissues of the parent 
root by a large number of strands. In the case of the phloem 
these connecting strands are very numerous, being often related 
to as many as half the total number of phloem-groups present 
in the parent root (Fig. 7). 
A transverse section near the base of a spine shows the 
phloem to be distributed around the periphery of the central 
cylinder in small groups isolated one from the other (Fig. 8), 
the xylem being restricted to two large lateral masses with 
smaller groups lying between them ; these latter are the first 
to fuse with the xylem of the main root. 
The examination of a series of sections from the base to 
the apex of a thorn shows that the phloem-strands travel in 
an irregular manner throughout the whole area of the stele. 
