290 MR. D. OLIVER, JUN. ON THE STRUCTURE OF THE STEM 
bundles to a great extent, by radial processes of cellular tissue, which may, in some cases, 
be regarded as projections from the medulla, probably in connexion with lateral organs; 
in others, as induplications of the cortical parenchyma. In Acanthophyllum spinosum it 
is often impossible, from the excessively dislocated character of the vascular bundles, to 
determine satisfactorily a true pith. In this species, the entire stem, almost to the young 
and leafy annual shoots, is partitioned into numerous vascular masses by tortuous in- 
truded plates of cellular tissue, presenting at first sight, in a transverse section, a striking 
similarity to the irregular structure of some species of Combretaceæ, Malpighiaceæ, or 
Bauhimie. The sole relation of the parts in these stems to a common centre is indicated 
more or less distinctly by some of the peripheral vascular bundles, which are laterally 
bounded and separated by lines of cellular tissue exhibiting a radial disposition. If we 
trace the structure by transverse sections from the young and yet green shoots, internode 
by internode downwards, we may observe the rapid accession of an irregular relative 
arrangement of the parenchyma and vascular tissues of the stem. Some of these changes 
are sketched in their consecutive order under fig. 3 (PL L.) ; figs. 4, 5, 6 and 7 represent 
also this curious and complicated structure. In the very young internodes the pith is 
found to be much elongated transversely, extending nearly or quite through the vascular 
zone, as shown by fig. 2. When dividing the wood entirely, as in fig. 4, the narrow ring 
of small and delicate cells, which in fig. 2 is represented as a normal * cambium" zone 
: surrounding the wood, penetrates the fissure and encloses separately the divided portions. 
Whatever may be the first determining cause of a deviation from the usual structure in 
this species, it is, I consider, obviously to the penetration of this belt of young and active 
cells that the subsequent and anomalous arrangement of the wood is due. These cells 
are doubtless capable of continuing the process of division and growth as under the 
ordinary conditions of the * cambium" layer, though probably to a limited extent. It 
may be observed, that in the very young shoots the direction of the transverse elongation 
of the pith may be found to alter materially from one internode to another. (See refer- 
ence to fig. 3.) . | 
The surrounding, intropenetrating *cambium zone’ is more or less apparent, not 
only in the earlier shoots, but also in portions of the stem which exhibit a highly dis- 
torted vascular system. In Acanthophyllum (Griffiths Affghan Coll. no. 1562) the 
dislocation of the wood is by no means so marked as in 4. spinosum, yet in this plant 
broad wedges of parenchyma are found to divide the vascular tissues into most unsymme- 
trical sectors, and sometimes extend quite across them. In À. laxiflorum these invasions 
are accompanied even to the apparent pith by cells containing the highly-coloured contents 
characteristic of the outer cortical layers. How far these wedges of cellular tissue may 
gen replace the medullary rays of Exogens generally, it would be interesting 
Fue. Singularly enough, although it was probably in Acanthophyllum (Gr. no.1562) 
that an irregular structure first caught my attention, I did not note or appreciate the 
