264 Scott and Brebner.—On Internal Phloem 
cambium in the older secondary xylem. Weiss regards them 
as the direct downward prolongation of the medullary leaf- 
trace bundles of the stem 1 . 
In the course of our work various facts of interest connected 
with bicollateral structure were observed in the stem as well 
as in the root, and beyond this, we were led to examine one 
plant ( Acantholimon ), which does not possess bicollateral 
bundles in the strict sense at all, though it has analogous 
peculiarities. Hence, our field of work is not very strictly 
defined. The present paper is only a beginning of the 
subject. Many important orders are entirely untouched, and 
there is clearly room for much further investigation, for which 
we have already some materials. 
The following observations then may be grouped under two 
heads : — 
1 . The relation between stem and root-structure in plants 
with bicollateral bundles. 
2. Special modifications of the stem-structure in plants, 
which belong to this category or present a similar arrange- 
ment of tissues. Observations closely connected with the 
subject before us will be found in the preceding paper on 
Ipomoea versicolor 2 , which for reasons of convenience has been 
dealt with separately. 
We adhere to the term bicollateral bundles in spite of the 
strong objections to its use which have been urged by Herail. 
This author rejects De Bary's terminology on the ground 
that it is only really appropriate to a single family, the 
Cucurbitaceae. In the majority of the plants in question, he 
finds that the medullary phloem does not appear at the same 
time as the rest of the bundle and does not proceed from the 
same meristem. It is especially on this second point, the 
independent position of the internal phloem, that he insists. 
His contention is supported by observations on the Solanaceae, 
Asclepiadeae, Apocynaceae, Myrtaceae, &c. 3 Without entering 
1 Markstandiges Gefassbiindelsystem, loc. cit., p. 406. 
2 See p. 173 of the present volume of the Annals of Botany. 
3 Herail, Etude de la tige des Dicotyledones j Ann. des Sci. Nat. Bot. S£r. 7, 
vol. II. 1885, p. 267. 
