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 meduUa, probably in connexion with lateral organs ; 

 in others, as indupUcations 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 Combrefacece, MalpighiacecB, or 

 BauUinice. 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 (PI. 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 

 surroimding 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 tliis 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 (Griffith's Affghan Coll. no. 1562) the 

 dislocation of the wood is by no means so marked as in ^. 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 A. 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 

 physiologically replace the medullary rays of Exogens generally, it would be interesting 

 to inquii-e. Singularly enough, although it was probably in Acanthojihyllum (Gr. no. 1562) 

 that an irregular structure first caught my attention, I did not note or appreciate the 

 absence in it of true medullary rays until my attention was called to it by Prof. Griese- 

 bach, to whom I showed a section. Their non-occurrence is an important circumstance, 

 and may be a key to a better understanding of the physiology of the ' systems,' so called, 

 of exogenous stems. Their absence I have since determined in other species of Accmtho- 



