and Histogeny of Strychnos. 293 
the leaf-trace bundles, which are therefore from the first 
of bicollateral structure. 
(b) These medullary groups grow by means of a special 
cambium lying on the outer side of each group. The 
formation of additional elements by the cambium results 
in the progressive obliteration of the older and effete 
portions of the phloem on the side towards the pith. 
3. (a) The phloem-islands , or interxylary phloem-strands, 
are formed centripetally by certain portions of the normal 
cambium. They become sunk into the wood owing to 
the relative retardation of the development of secondary 
xylem on their inner side. The wood ultimately closes 
round them in consequence of the formation of a comple- 
mentary cambium, which arises from the division of cells 
of the external bast-parenchyma, and thus again com- 
pletes the cambial ring. 
(b) The phloem-islands continue to grow, after they are 
enclosed in the wood, by means of the cambial layer on 
their inner side. This growth is opposite in direction 
to that of the medullary groups, and results in the ob- 
literation of the older tissues occupying the outer part of 
the island. 
4. The roots, in so far as they have a pith, possess medullary 
phloem-groups, similar to, but smaller than, those of the 
stem, and increased, like the latter, by means of a centri- 
fugally active local cambium. 
Comparative Considerations. 
In possessing bicollateral bundles Strychnos agrees with a 
large part of the Loganiaceae 1 , and with many other natural 
orders of the most diverse relationships, among which the 
Myrtaceae, Onagraceae, Lythraceae, Cucurbitaceae, Melastom- 
aceae, Solanaceae, Asclepiadeae, Apocyneae, Cichoriaceae, 
and Campanulaceae may be mentioned. This is not the place 
to discuss the systematic importance of this character, a 
1 See Solereder, 1. c. pp. 28 and 176. 
X 2 , 
