266 Scott and Brebner . — On Internal Phloem 
stem (in which the secondary wood averages six elements in 
radial thickness) shows a ring of numerous small groups of 
internal phloem, each group of about the size of one of the 
smaller cells of the pith. All the groups lie at some distance 
within the protoxylem of the bundles, from which they 
are separated by at least two parenchymatous cells, usually 
more. The phloem in the pith is collectively greater in 
amount than that in the normal position. The medullary 
groups are entirely confined to the outer small-celled zone of 
pith. In an older stem, with wood about thirty cells thick, the 
medullary phloem-groups have increased somewhat in bulk, 
divisions taking place in the adjacent cells so as to add new 
elements to the phloem. There is, however, no regular 
internal cambium in this plant and the groups remain 
isolated. The external phloem receives slight additions 
from the normal cambium, but its proportion to the internal 
phloem is little affected. It is worth pointing out incidentally, 
that the stem has a beautifully clear endodermis with the 
characteristic dots on the radial walls. This is continuous, 
through the hypocotyl, with the endodermis of the root. 
The main root is diarch and shows perfectly typical struc- 
ture, without any pith (see Fig. 5). Sections of older roots 
show that secondary thickening goes on in the usual manner. 
This, then, is a plant in which the medullary phloem of the 
stem has completely disappeared in the root. It remains to 
trace its downward course and to determine its relation to 
the normal phloem-groups of the vascular cylinder of the 
root. The transverse section of the hypocotyl shows six 
bundles. Of these, two opposite pairs are the cotyledonary 
traces (see Fig. 1). The bundles of each pair have their 
protoxylem-groups turned towards each other. The two 
remaining bundles, lying between the two cotyledonary pairs, 
proceed from the leaves above the cotyledons. They dis- 
appear lower down, anastomosing with the cotyledonary 
traces. It is the latter only which are directly continuous 
with the xylem of the root. The pith is small compared to 
that of the epicotyledonary stem. It contains a number 
