( Intraxylary) Phloem in the Stems of Dicotyledons. /. 569 
anastomose with the latter; after fertilization they become obliterated, 
and the conduction of plastic substances is thereafter carried on by the 
usual phloem parts. 
Herail states that the Cucurbitaceae are the only plants which have 
bicollateral bundles, for the development of all three parts of the bundle is 
identical and synchronous. He noted in Zanonia sarcophylla the occurrence 
of interfascicular cambium connecting the bundles of the outer and inner 
rings into a single ring. 
Lamounette has an interesting thesis on the morphological origin 
of the internal phloem in Cucurbita maxima . He found that in the region 
between the c heel * and the first rootlets of the young hypocotyl the 
external phloem had developed considerably, while the internal phloem was 
being separately initiated by a few divisions in the parenchyma of the pith. 
No communication was observed between the two. The formation of the 
internal is subsequent to that of the external phloem. The above applies 
also to Cucumis and Liiffa. He concludes (not only for Cucurbitaceae but 
for other orders investigated) that internal phloem is an abnormal formation 
due to the activity of certain cells of the central conjunctive parenchyma, or 
is the result of the ulterior evolution of these cells ; it has been acquired 
during evolution and then transmitted by heredity. In the cotyledon and 
leaf of Cucurbitaceae the internal is also of later formation than the external 
phloem ; it does not pertain to the procambium, but has a distinct evolution 
from the parenchyma. He says the term ‘ bicollateral ’ should be abolished 
in view of the origin of the internal phloem. The Cucurbitaceae afford the 
best example of the acquired secondary dependence of the internal phloem 
on the bundle of the ring ; its more primitive condition is as an independent 
bundle in the pith. 
Scott and Brebner found in Thladiantha dubia that the internal 
phloem connects with the external in the medullary ray. In a valuable 
study of the course of the medullary phloem in plants generally, they 
found that this tissue, during the transition from stem to root, passes out 
and unites with the external phloem. This agrees with what has been 
observed in Lagenaria in the present paper. 
Flot, like Lamounette, will give no quarter to the term c bicollateral ’, 
in view of the fact that the internal phloem arises independently from the 
perimedullary zone. 
Baranetzky found in the stem of Rhynchocarpa dissecia that an inverted 
medullary collateral bundle which ran through one internode and part 
of another, was separated from the bundle of the ring by two or three layers 
of medullary parenchyma. Some only of the medullary bundles possessed 
xylem, and this usually died out in some region of the internode in following 
the bundle either upwards or downwards. 
In Bryonia dioica and Zehneria suavis the same facts with regard 
P p 2 
