I 



IN CERTAIN SPECIES OF CARYOPHYLLEyE AND PLUMBAGINE^. 291 



phyllwm, in Dianthua {D. hiapcmicus) and Arenaria {A. laricifolia), as also in Armeria in 

 Plumbaf/inece. Schacht* records their absoDce in the 'rhizome' of Viola {V. mirabilis 

 and odorata), and A. Brongniart in certain CrassiilacecBf. It is probahlfe 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 Acanthophylliim spmosum, 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 imable to satisfy myself entirely as to 

 the occasion of these alternatiag 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 concentric 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 ^. spinosimi I have not observed any of the 

 tissue to become thus thickened. This recurring deposition of spu*al 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 spii-als thus repeated is common to all species 

 destitute of medullary rays. The inquiry is an important one, and doubtless of physio- 

 logical value. In the older stems of A. spinosum, in which no concentric disposition of 

 the elements of the vascular masses is apparent, I should observe that the spirals seem 

 to be almost indiscriminately scattered amongst the wider and ' slit-marked ' vessels. In 

 the other species of Acanthophyllwm examined, and in Arenaria laricifolia, I am not sure 

 that true spirals recur in the wood, in the maimer 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). 



X I use the term ' slit-marked ' as best expressing the form presented by their pits, which are transversely more or 

 less lengthened. 



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