316 NATURAL HISTORY OF PLANTS. 



fibrovascular bundles, standing in definite relations with the leaves. 

 In several Crassulas the absence of a woody zone has been de- 

 monstrated. The bundles of the medullary layer, consisting of 

 tracheae and spiral annular and reticulate ducts, increase in number 

 and volume, without producing a woody parenchyma. In several 

 Cotyledons* the wood has been seen formed of fibres only, free from 

 vessels, and traversed by vertical bands of parenchyma consisting of 

 thin-walled cells, between which spiral vessels are included. When 

 the Crasmlacece become woody, as in Sempervivum arboreum* for 

 instance, a stem several years old possesses a cortical cellular zone, 

 with a suberous layer outside, and a herbaceous inside. The wood, 

 formed of dotted thick-walled fibres, is traversed by scattered isolated 

 vessels. In the thickness of the wood, untraversed by true medullary 

 rays, are arranged peculiar concentric zones, formed of vessels and 

 elongated quadrangular cells with a delicate unsculptured wall. 

 Bundles formed of annular and unrollabio spiral vessels, with 

 elongated cells, occur in the wood all round the pith, which 

 presents no peculiarity of structure. The same fundamental 

 type, modified in details only, recurs in the other woody species 

 belonging to Crassula, Rochea, Cotyledon, &c. The most general 

 characteristics of the order 3 are the absence of liber and medullary rays 

 in the adult wood ; the presence of cords formed of fibroid cells and 

 vessels, corresponding with isolated parts of the generative zone, 

 within the body of the wood ; and the predominance of the paren- 

 chymatous tissue, giving the organs their peculiar fleshy succulent 

 consistency. 



These peculiarities, and yet more the outward appearance due to 

 the fleshy organs, have at all times induced a comparison between 

 the Crassulacete and other succulent plants, especially Cactacete and 

 MesembryanthemacecB. But they differ in their carpels, as remarked 

 by A. L. de Jussieu. 4 Hence he placed them in Poly petal a 

 Periyyna, next Saxifrayacea. Penthorum, made by him a yenas affuie 



1 Link., Icon. Sel. (1839), fasc. 1, vii. 1,2; 2 Regnattlt, Rech. sur les Affin. de la Tige 



in Wiegm. Arch. (1839), 224; in Ann. Nat. des PI. du Or. des Cyclospermees (in Ann. Sc. 



Hist., iv. 241. — Lindl., Yeg. Kingd., 345. In Nat., sen 4, xiv. 87). 



Sedum the structure of the stem is much nearer 3 Oltv., Stem, in Dicot., 16. (See also P. 



that of normal Dicotyledons. The tuberous roots, Magnus, in Pot. Zeit. (1871), 480. 



however, present certain histological peculiarities 4 Gen., 308 : " utpole polygynm." 

 (Heney, Ueb. die Bildung d. Wurzelfasern v. 

 S. Telephium, in Verli. Nat. S., v. (1860, 61). 



