6o8 SECONDARY CHANGES. 



roots of Sedum Telephium and its allies is traversed by vascular bundles, which are 

 arranged in the transverse section in a ring. In many species, e.g. Sedum Fabaria, 

 these bundles are connected throughout by a simple, normal cambial ring, which is but 

 slightly productive. This is found sometimes also in S. Telephium, and even in certain 

 roots of those stocks, which otherwise show the condition now to be described. This, 

 which is the rule in S. Telephium, is as follows. At the point of insertion of the root a 

 simple, normal, smooth, cambial ring connects the bundles ; further between each pair of 

 bundles the ring curves towards the swollen centre of the root, and the more deeply the 

 nearer to the apex, till the stage is reached at which it is divided at the centre into as 

 many separate partial rings as there were inflections or original vascular bundles. The 

 separate rings are arranged in the transverse sections in a circle, and are separated one 

 from another by bands of parenchyma. Each has a closed, and but slightly productive 

 cambial layer, which both on the side next the wood and that next the bast forms especially 

 parenchyma ; on the side of the wood facing the middle of the partial ring are isolated 

 small groups of vessels, on the side facing the bast, and corresponding to the latter, are 

 those small groups of narrow elements of soft bast mentioned on p. 324. Towards the 

 apex of the root the partial rings open, and unite again to form a normal xylem body of 

 the root. Comp. Koch, /. t. 



Sect. 195. The structure of the stem of the Cycadecs'^ must here be dealt with 

 generally, in connection with p. 256, since a separation of the primary 'arrangement 

 from the various secondary changes could not be well carried out without too great 

 sacrifice of clearness. 



By way of making matters plain, it may be stated at once that the stem always 

 has at first the same arrangement of tissues as that typical of the Dicotyledons 

 and Gymnosperms ; i. e. it has a normal ring of wood, bast, and cambium, which 

 separates the outer cortex from the pith. The two last-named regions are always 

 relatively strong ; in the old stem of Cycas circinalis, investigated by Miquel, 

 for instance, the outer cortex was more than 7 cm j^ thickness, and the thickness 

 of the cylinder of pith 10 cm. They consist like the medullary rays of relatively 

 thin-walled, permanently starchy parenchyma, and are traversed by the system 

 of branched gum and mucilage canals, described above on p. 441. The cortex 

 is covered by a superficial layer of periderm. The two kinds of leaves are, as is well 

 known, arranged spirally, and are so close, one above another, that the fleshy 

 scale-like expanded bases of the leaves are in superficial contact one with another. 

 The numerous vascular bundles of the base of the leaf are united at the surface 

 of insertion into two bundles of the trace, which enter the stem separately and 

 symmetrically near the middle of the surface of insertion ; they then diverge directly. 



shown that the ring of bundles now under consideration is nothing more than the very isolated vascular 

 groups of a chiefly parenchymatous xylem of the root, which is derived from a typical radial root- 

 bundle, the original elements of which had hitherto been overlooked in the massive parenchyma. 

 The napiform roots of Sedum are thus a special case of very parenchymatous typical dicotyledonous 

 roots. What is said above should accordingly be corrected. 



1 A. Brongniart, Rech. sur 1' organisation de la tige des Cycadfes ; Ann. sci. nat. I ser. XVI. p. 

 369 ; Id. Archives du Museum, I.— von Mohl. Ueber d. Bau d. Cycadeenstamraes ; Abh. d. Munchn. 

 Acad. I. p. 397 ; Verm. Schriften, p. 195. — Miquel, Ueber d. Bau e. Stammes, &c. von Cycas circi- 

 nalis ; Linnsea, Bd. XVIII. p. 125.— Karsten, Organogr. d. Zamia muricata, in Abh. d. Berliner 

 Acad. V. 1856, p. 193.— Mettenius, Beitr. z. Anat. d. Cycadeen; Abhandl. d. K. Sachs Ges. d. Wis- 

 sensch. VII. p. 567. 



