52 Thompson—The Structure and Development of 
tion stages, for two bundles have acquired internal phloem, 
while two bundles are as yet devoid of it. The region of the 
first leaf node shows the acquired condition of the presence 
of internal phloem in all four bundles. 
The physiological significance of this acquisition, and the 
causes that led to it, are not clear. It is a noteworthy fact 
that internal phloem appears only in parts of this plant where 
pith is present. Although present in the stem, internal phloem 
is absent throughout the greater length of the petiole. It is 
present in the upper portion of the hypocotyl, but is absent in 
the lower part where the pith area is becoming constricted by 
inward growth of the xylem. Both internal phloem and pith 
are absent in the roct. In plants like S¢rychnos, whose roots 
possess medullary phloem, pith is always present. 
The view may be advanced, that to utilize the pith area, 
either for more perfect protection of the phloem, in these twisted 
and at times contorted stems, or to increase the total amount 
of it, a portion of the external phloem, during the evolution of 
the plant, dipped in from the bases of the petioles, through the 
fissures formed by the leaf traces in the vascular cylinder, and 
became internal in position. The climbing habit of this plant 
may be one of the factors in its evolution. 
SUMMARY OF RESULTs. 
1. The internal phloem arises primarily as four longitudinal 
strands, which are an integral part of the leaf trace bundles. 
2. The origin of the internal phloem is simultaneous with, 
or slightly later than, the protoxylem and external phloem, so 
that the leaf trace bundles are bicollateral from the first. 
3. The internal phloem patches are bounded internally by 
a two-celled phloem sheath. 
4. The internal phloem patches grow centrifugally by 
means of a medullary cambium, the inner and older layers 
in time becoming crushed and obliterated. 
5. Death of the pith occurs early in the first year. 
