and Histogeny of Strychnos . 297 
small phloem-groups which are found in the parenchymatous 
islands is a secondary, or rather a tertiary process, like that 
of the interxylary bundles in the fleshy roots of Cruciferae, 
described by Weiss \ That is to say, certain of the secondary 
parenchymatous cells divide up to form a group of sieve-tubes 
and companion cells, which then increase in number by means 
of a special cambium surrounding the group on all sides. This, 
however, does not alter the main fact ; the interxylary phloem 
of the root, as well as of the stem, is developed, whether 
directly or indirectly, on the inner side of the normal cambium 2 . 
The accuracy of Kolderup-Rosenvinge’s figures renders any 
illustrations of our own unnecessary. 
As regards the Loganiaceae, we need only mention that 
Solereder found phloem-islands in two additional genera, 
Norrisia and Antonia. 
In Chironia , one of the Gentianeae, Vesque 3 found numerous 
small interxylary phloem-islands. From his description and 
figures it is most probable, though not quite certain, that they 
are formed on the inner side of the cambium. 
In Thunbergia and in Hexacentris (perhaps a sub-genus of 
the former) among the Acanthaceae, Vesque 4 found alternating 
tangential bands of phloem and xylem, the origin of which 
was subsequently more fully investigated by Herail 5 . He finds 
that this abnormality depends on the unequal activity of the 
cambium. Opposite the primary xylem-bundles the cambium 
forms abundant wood containing vessels, while between them 
it forms a smaller amount of wood destitute of vessels, but is so 
much the more active in the centripetal formation of phloem- 
id ence the cambium shows a depression in each of these latter 
1 Anat. u. Physiol, fleischig verdickter Wurzeln, in Flora, Bd. lxiii, 1880. 
Compare especially his figures 1 and 2, of Cochlearia . 
2 Treub describes the formation of secondary vascular bundles from cells of the 
mature parenchyma in Myrmecodia, in which the primary bundle system perishes 
on the formation of the first internal periderm. From the account he gives it 
appears that the bundles may be formed from parenchyma, which is itself secondary 
(phelloderm), as well as from the primary ground-tissue. Treub, Sur le Myrmecodia 
echinata , in Ann. du Jardin bot. de Buitenzorg, T. iii, 1883, p. 139 ; PI. xxii, Figs. 
3, 4 ; xxiv, Figs. 8, 9, 10. 
3 1. c. p. 147. 4 1. c. p. 147. 
5 1. c. pp. 259-263. 
