Internal Phioem in Ge.sentuum sempervirens, Ait. 51 
ORIGIN OF THE INTERNAL OR MEDULLARY PHLOEM. 
In the growing apex of the stem the first cells to differen- 
tiate from the primary meristem are the spiral trachez of the 
protoxylem, which are arranged in radial rows. On their outer 
border appear groups of very small, thin walled cells, whose 
division walls lie in all planes. Soon thereafter similar groups 
of small cells are differentiated on the border of the pith area. 
These represent the internal phloem patches. The course of 
the internal phloem has been traced in older stems into the 
petioles, so it may be regarded as an integral part of the 
leaf trace bundle. It owes its origin to the same primary 
meristem that gives rise to the external phloem, and the pro- 
toxylem. Certain primary meristematic cells on the inner face 
of the protoxylem represent the medullary cambium. To 
the later activity of these cells the secondary growth of the 
medullary phloem is due. A radial arrangement of the later- 
formed medullary phloem cells is to be observed, and is an indi- 
cation of their cambial origin. The medullary phloem appears 
in the hypocotyl some time after the differentiation of protoxy- 
lem and external phloem. Its origin, however, is from embryo- 
nic cells that area part of the original primary meristem of the 
bundle. The appearance of these embryonic cells, on the 
inner side of two bundles in the hypocotyl, at a definite 
point below the cotyledonary node, and of similar cells in the 
two opposite bundles in the epicotyl, just below the first leaf 
node, may be explained as follows on phylogenetic grounds. 
Internal phloem is a secondary character acquired during the 
evolution of the plant. Since the hypocotyl and cotyledons 
are embryonic structures representing the primitive stages of 
growth of the plant, characters that have been acquired by, 
and are adapted to, the adult stem, may reasonably be found 
absent throughout the whole, ora part, of the hypocotyl. 
In this plant the lower portion of the hypocotyl exhibits the 
ancestral condition in the absence of internal phloem. The 
upper portion of the hypocotyl! and of the epicotyl are transi- 
