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 

 annvilar 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 a few of them being radiaUy approximated. The thinner tissue which separates 

 these annular zones is extremely narrow, and with difficulty resolvable from its ready 

 ruptm-e in cutting the adjacent ceUs, the cavities of which are nearly obliterated by 

 secondary hardened deposits. In A. laxiflorim the arrangement of the thick-walled 

 tissue is perhaps yet more cm*ious. 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 ' sht-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 u-regular 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 wUl be apparent, from what has been stated, 

 that Acanthophyllum (Gr. no. 1562), A. laxifiorum, and Acanthophyllum (Gr. no. 1570), 

 exhibit their development in a sort of graduated series, while in A. spinosum they are, so 

 far as I have observed, totally absent. The singular prevalence in some of the species of 

 Acanthophyllum 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 occur 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 increase 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. 



PlUMBAGINE^. 



The stems of Armeria maritima, Acantholi/mon 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 u-regular ly traversing the vascular 

 bundles radially, from three or four of them to a considerable number sometimes occur- 



