in the Root and Stem of Dicotyledons . 269 
origin (see for example Fig. 3), and in other cases the im- 
bedded phloem-group is connected with the cambium by 
thin-walled tissue, as is shown clearly in Fig. 4. 
Other sections through the transitional region of an older 
plant, with a thick zone of secondary wood, showed phloem- 
groups imbedded in the latter, at a distance from the primary 
tissues which left no doubt as to their origin from the cam- 
bium. We conclude, therefore, that the phloem of the root- 
system remains in permanent, and not merely in transitory, 
connection with the medullary, as well as with the external 
phloem of the sub-aerial organs. 
As regards the main points Browallia may be taken as a 
type of those plants which have medullary phloem in the 
stem, but have normal roots. Further information on the 
points discussed will be found in the paper on Ipomoea versi- 
color. Our observations agree essentially with those of Gerard, 
above cited. He traced the transition in the opposite direction 
namely from root to stem. 
2. Asclepias obtusifolia , Mx. The Asclepiadeae are among 
the plants in which the bicollateral structure is best known 
So far as the stem is concerned the above species presents 
nothing remarkable. The internal phloem is in the form of 
a ring of very numerous groups, which remain isolated from 
one another. They are not localized exclusively opposite 
the protoxylem-groups, but also border on the interfascicular 
tissue. They are generally separated by two or three paren- 
chymatous cells from the xylem-elements. The internal 
phloem, although not so great in amount as in some other 
plants, is extremely well developed, the sieve-tubes and the 
companion-cells being large and characteristic. There are 
occasional divisions in the cells bordering on the groups 
externally, but no considerable cambial increase takes place. 
Petersen’s 1 Fig. 26 (from A. pr triceps') gives a fair idea of the 
structure of one of these groups, but in our plant there are 
more sieve-tubes in each. The diameter of a sieve-tube is 
often as much as ’03 mm. 
1 Loc. cit., PI. VII. 
