IN CERTAIN SPECIES OF CARYOPHYLLEÆ AND PLUMBAGINE E. 291 
phyllum, in Dianthus (D. hispanicus) and Arenaria (A. laricifolia), as also in Armeria in 
Plumbagmee. Schacht* records their absence in the ‘rhizome’ of Viola (F. mirabilis 
‘and odorata), and A. Brongniart in certain Orassulaceæt. It is probable that a wood 
destitute of these processes may prevail through other natural orders in which a similar 
perennial, depressed axis is found. 
In very young internodes of Acanthophyllum spinosum, and in the stem of Acantho- 
phyllum (Gr. no. 1562), more or less regular concentric zones are apparent in the woody 
mass. These vary in number and width. I am unable to satisfy myself entirely as to 
the occasion of these alternating zones. They may, in these species, result, as might be 
expected, from the annual alternation of seasons; but upon this head further evidence is 
required. In the younger branchlets of A. spinosum they are often distinctly perceptible ; 
in the older portions, from the advanced distortion of the tissues, they are nearly or 
. quite lost. 
The change, from whatever cause it may result, in the character of the vascular 
bundles which gives rise to these concentrie rings, is of much interest, and I think im- 
portant to the phytotomist. I feel tolerably satisfied that in Acanthophyllum (Gr.no.1562) 
and A. spinosum, the vascular mass, which consists in great measure of very numerous 
‘ slit-marked’{ vessels of various calibre, traversing a prosenchymatous tissue, is inter- 
rupted by the formation of narrow annular belts of spiral vessels of small diameter 
repeated at definite (?) intervals, and that, in the young shoots of A. spinosum at least, the 
concentric zoning is due to these. In Acanthophyllum (Gr. no. 1562) numerous cords 
of a thick-walled prosenchyma occur along with the vessels, and these probably assist 
in determining the annular formations: in A. spinosum I have not observed any of the 
tissue to become thus thickened. This recurring deposition of spiral vessels is, I be- 
lieve, a repetition periodically of that ring in which they occur, and which is so generally 
recognizable, in Dicotyledons, immediately, and only, around the pith, and which, from 
the peculiar character which the presence of these vessels confers, is usually distin- 
guished by the special term ‘medullary sheath.’ It remains yet to be ascertained, and I 
have not leisure to follow up the investigation myself except to the neglect of other 
studies, whether an annular formation of spirals thus repeated is common to all species 
destitute of medullary rays. The inquiry is an important one, and pilen arr Bon 
logical value. In the older stems of A. spinosum, in which no concentrie disposition of 
the elements of the vascular masses is apparent, I should observe that apaapitels seem 
to be almost indiscriminately scattered amongst the wider and * slit-marked vessels. In 
the other species of Acanthophyllum examined, and in Arenaria laricifolia, lam not sure 
that true spirals reeur in the wood, in the manner described; nor am I quite clear that 
they do so in Dianthus. 
In the case of Acanthophyllum (Gr. no. 1562), allusion has been made to the presence 
of cords of much-thickened prosenchymatous cells, which traverse, with the vessels, the 
thinner tissues of the woody masses. In this plant these thickened cords are, I think, 
* «Die Pflanzenzelle,’ p. 280. f Arch. Mus. i. 437 (Lindl. V. K. p. 344). 
. $ I use the term ‘slit-marked’ as best expressing the form presented by their pits, which are transversely more or 
less lengthened. 202 
