Io BOTANICAL GAZETTE [JANUARY 
protecting the phloem. Separating the phloem from the xylem is 
a well marked cambial region, characterized by the regularly 
seriate arrangement of its cells. Fig. 17 represents the foliar 
trace of fig. 15, under the same magnification as fig. 16. It is 
clear that the cambial activity found in the stem bundle is con- 
spicuous by its absence in the leaf trace, since there are no 
regularly radially arranged cells intervening between xylem and 
phloem. 
Before discussing the significance from the evolutionary stand- 
point of the facts elucidated in figs. 15-17, it will be well to consider 
the conditions present in the bundles of the root, using Aster, 
since this genus presents a more illuminating range of organization 
in the root than that which characterizes Helianthus. Fig. 18 
represents a transverse section of the mature root of Aster Shortit. 
Root hairs are conspicuous by their absence, and the strong develop- 
ment of the secondary wood is very obvious. In the midst of the 
secondary organization of the wood can be seen a small 5-angled 
star of primary xylem, the points of which alternate with the 5 
dark hued masses of phloem seen on the margin of the fibrovascular 
cylinder of the root. Fig. 19 is a transverse section of a young root 
of the same species of Aster, in which the secondary growth of the 
xylem is just beginning. The primary xylem star with its 5 points 
alternating with 5 angles of phloem can distinctly be seen. Root hairs 
are present on the surface of the root, but are beginning to disappear, 
as is normally the case in roots in which secondary structures 
have begun to form. Fig. 20 represents a very different type of 
root, in which the organs are distinguished by the indefinite per- 
sistence of the root hairs. This condition is well shown by the . 
varieties of the somewhat inconstant species Aster umbellatus. 
Here the roots continue as hirsute structures for several years, and 
in fact this condition persists until they have become quite dark 
with age and have begun to decay. Fig. 20, which represents an 
old root of this species, shows that the persistence of the root hairs 
is paralleled by the absence of secondary structures in the cylinder 
of the organ. This is an interesting general condition of the 
organization of permanently hairy, and, as a consequence, pre- 
sumably persistently absorbent roots. 
