2 73 
in the Root and Stem of Dicotyledons . 
Weiss in various Onagraceae, Gentianeae, and Solanaceae 1 . 
He, however, found that the interxylary phloem was always 
formed directly from the cambium, and that the groups never 
increased subsequently by a special cambium of their own. 
In Asclepias , as we have seen, phloem-islands may be formed 
in tissues far removed from the cambium, while the groups 
constantly show cambial growth themselves, though not on 
a great scale. The sharp distinction, which Weiss endeavours 
to draw between his interxylary bundles and interxylary phloem 
so far falls to the ground. 
3. Thladiantha dubia , Bunge (Cucurbitaceae). We take 
this plant next, because, though systematically so remote from 
Asclepias , it presents certain analogies with that genus as 
regards the points under consideration. The bundle-system 
of the stem is of the usual Cucurbitaceous type and is shortly 
described by De Bary 2 . The internal phloem, which is not 
very greatly developed, is connected with the external mass 
by sieve-tubes in the medullary rays, as happens in other 
Cucurbitaceae 3 . There is a distinct interfascicular cambium 4 . 
Some of the roots 5 have very curious tuberous swellings, to 
the structure of which we will return. But first the more 
ordinary root-structure must be described. The bundle- 
cylinder is most often tetrarch, but this is not constant, and 
indeed the same root may pass from hexarch to tetrarch 
structure in various parts of its course. The roots have no 
pith, the primary xylem-bundles often meeting in one or two 
large central vessels. A group of bast-fibres lies outside each 
of the primary phloem-masses, as in the Leguminosae. As 
the root grows in thickness the bast-groups break up into a 
1 Loc. cit. Flora, 1880, p. 97, and Bot. Centralblatt, XV. p. 408. 
2 Loc. cit., p. 248. 
3 De Bary, loc. cit., p. 338. We have occasionally found a few small sieve-tubes 
in the secondary rays also. 
4 Cf. M. C. Potter, On the increase of the thickness of the stem of the Cucur- 
bitaceae ; Proc. Cambridge Phil. Soc. vol. VII. p. 4. 
5 Some of the peculiarities of these roots have already been described by Dutailly, 
Sur quelques phenomenes determines par l’apparition tardive d’elements nouveaux 
dans les tiges et les racines des Dicotyledones. Paris, 1879, pp. 37 and 39. 
