292 MR. D. OLIVER, JUN. ON THE STRUCTURE OF THE STEM 
sometimes considerably larger than I have represented in fig. 13. This remarkable 
thickening of the prosenchyma attains a maximum, however, in Acanthophyllum (Gr. 
no. 1570), in which its extensive development is the conspicuous feature of the wood, 
Here it is not disposed in cords or isolated masses, but is nearly continuous through each 
annular zone, although with many sinuosities in its cross section, traversed by the vessels, 
which are not very numerous, and chiefly confined to the inner portion of the rings, here 
and there à few of them being radially approximated. The thinner tissue which separates 
these annular zones is extremely narrow, and with difficulty resolvable from its ready 
rupture in cutting the adjacent cells, the cavities of which are nearly obliterated by 
secondary hardened deposits. In A. laxiflorum the arrangement of the thick-walled 
tissue is perhaps yet more curious. It is here remarkably dense, and is disposed, not in 
continuous belts, but in large cords, which are singularly lobed and divided in their hori- 
zontal section. The tissue filling up their winding interstices is almost entirely made up 
of ‘slit-marked’ vessels often of conspicuous diameter. The vertical continuity of these 
vessels through so dense a prosenchyma is interesting and readily observable in a thin 
longitudinal slice, the transverse septa of the cells of which they were originally consti- 
tuted frequently remaining quite distinct. The irregular form of these masses does not 
interfere with the regular arrangement of the annular zones within which they are 
confined. | | 
In respect to this thick-walled tissue, it will be apparent, from what has been stated, 
that Acanthophyllum (Gr. no. 1562), 4. laxiflorum, and Acanthophyllum (Gr. no. 1570), 
exhibit their development in a sort of graduated series, while in 4. spinoswm they are, so 
far as I have observed, totally absent. The singular prevalence in some of the species of 
Acanthophylium of an unusually large deposition of crystals of oxalate of lime is an addi- 
tional item of interest worthy of remark. I am indebted to my kind friend Daniel Han- 
bury for the accurate determination of these crystalline concretions from, I think, Acan- 
thophyllum (Gr. no. 1562). They oceur abundantly scattered through the parenchyma 
of the stem, both in the inner cortical layers and its inversions, the irregular cellular 
plates which radially traverse the wood, and also in the pith. | | 
In Acanthophyllum spinosum they are small, and do not probably much inerease be- 
yond the boundary of the cell in which they originate. They are very numerous, accom- 
panying the parenchyma in its twistings through the stem-structure,—in the vertically 
elongated cellular tissue being often also lengthened, or rod-like. In Acanthophyllum 
(Gr. no. 1570) they are comparatively few, but chiefly very large, without doubt obli- 
terating several cells in their increase. The concretions of Acanthophyllum (1562) are 
very variable in size and extremely numerous,—the parenchyma, laid bare by removal of 
the outer cortical layers, being quite gritty from their presence. 
PLUMBAGINEA. 
The stems of Armeria maritima, Acantholimon diapensioides, and, I think also, Statice 
arborea, present a wood destitute of medullary rays. Parenchymatous processes which 
may be accompanied by a few vessels are found very irregularly traversing the vascular 
bundles radially, from three or four of them to a considerable number sometimes occur- 
