in the Root and Stem of Dicotyledons . 263 
structures. Weiss in his second work lays stress on the 
distinction between medullary phloem and medidlary bundles. 
The former is simply the internal part of the phloem in the 
bicollateral bundles. The medidlary bundles (which possess 
or may possess xylem as well as phloem) are an independent 
system and usually represent the lower ends of the leaf- 
traces, which have already passed down one or more inter- 
nodes as part of the normal ring. The distinction, plain 
enough in the extreme cases, is by no means obvious in all, for 
on the one hand the medullary phloem of bicollateral bundles 
may be widely separated from its xylem, while on the other 
hand the medidlary bundles often consist of phloem-strands 
only. Weiss states that the medidlary phloem as distinguished 
from the independent medullary bundles may be recognized 
by the following characters : — 
1. The medullary phloem-strands m Solanaceae, Ascle- 
piadeae, Apocynaceae, Gentianeae, Convolvulaceae, Vochysia- 
ceae, Cucurbitaceae, &c., in every case accompany the leaf- 
traces, within which they lie, on their exit into the leaf. 
2. They arise almost at the same time with, or only a little 
later than, the parts of the phloem outside the xylem. 
3. Where a cambium forms in connection with them, it 
never produces wood also, as it does in the medidlary bundles 
of some of the Cichoreaceae and Campanulaceae, and in 
Tecoma , Acanthus, &C. 1 . 
This last distinction (3) was originally due to Vesque 2 . It is 
a merely empirical generalization and, as might have been 
expected, has proved not to hold good. The other differences 
are of value, though their application is not always easy. 
Plants with medidlary bundles in the stem may also have 
interxylary phloem in the root, as in various Cruciferae (e. g. 
Cochlearia). In the typical cases each phloem-strand in the root 
is surrounded by xylem-elements, forming a tertiary concentric 
bundle. These structures arise at a distance from the normal 
1 Markstandiges Gefassbundelsystem. loc. cit., p. 396. 
2 Loc. cit. Vesque speaks of false cambium in these cases. See our subsequent 
observations on Apocynam and Willughbeia . 
U 
