SECONDARY THICKENING. NORMAL DICOTYLEDONS. 499 



pubescens ; or they form with the parenchyma extensive groups, constituting trans- 

 versely elongated, occasionally irregular, interrupted transverse bands — Semper- 

 viviim arboreum. Lastly, in the small creeping stems of Sedum reflexum, they form, 

 together with the delicate cells, thick, multiseriate, continuous annular zones, which 

 alternate with similar fibrous zones. Other species however, even those with thick 

 stems, as Crassula lactea, Sedum ternatum, and Echeveria pubescens, form mere 

 traces of secondary wood, which are scarcely worth mentioning. 



In the Caryophyllecs investigated^ (Dianthus, Gypsophila, Silene spec, and Arenaria 

 graminifolia), especially in their rhizomes, thin-walled, long-celled parenchyma, often 

 forming large irregular islets or annular segments, is inserted between fibrous masses 

 of similar form, but in the foliage stems it may be absent, e. g. Gypsophila altissima. 

 The vessels are numerous, and occur among both forms of tissue, often forming 

 interrupted radial rows ; in the fibrous groups they are often unaccompanied by 

 parenchyma. In the foliage shoot of Dianthus plumarius the main mass of the very 

 slight secondary thickening consists of vessels. 



In the stem oi Rurnex Lunaria, the pitted vessels, accompanied by rows of bundle- 

 parenchyma or by intermediate fibres, lie in very regular but interrupted radial 

 rows in the fundamental mass, and the latter consists of fibrous cells densely filled 

 with large starch-grains. With the exception of the starch, the same holds good 

 for Centradenia grandifolia (comp. p. 484). In Campanula Vidalii the vessels are 

 very isolated, and scattered in the radial series of fibrous cells, while I was unable to 

 find any parenchyma or intermediate fibres accompanying them. 



The Rhinanihacece require further investigation. 

 f The secondary wood of the thicker, fleshy stems of CaciecB "^ must again be men- 

 tioned here. The peculiar structure of the ligneous bundles, vessels, and tracheides, 

 in the Mamillarise, Echinocactus, &c., has already been repeatedly noticed above 

 (comp. p. 478), and it must here be added, that in the genera mentioned the whole 

 secondary ligneous bundle consists of the spirally and annularly thickened tracheae, 

 usually W'ith a deeply projecting fibre, between which very thin-walled bundle- 

 parenchyma is imbedded, in single longitudinal rows. The medullary rays of all 

 ranks resemble the latter as regards the delicacy of their cells. In other Cacteae, 

 especially species of Cereus and Opuntia, the secondary ligneous bundle consists of 

 tough woody fibres, and the scattered reticulated vessels which are usually accom- 

 panied by parenchyma (see p. 479). The parenchyma, including that of the medul- 

 lary rays, may be very thick-walled and lignified, e. g. in Cereus speciosissimus. 

 In the Opuntise short tracheides are also present, with the above-mentioned 

 thickenings on the wall, which project inwards in the form of plates, and are usually 

 annular. These elements are sometimes distributed singly in the interior of the 

 bundle (O. tunicata, O. robusta), sometimes at its edges (O. cylindrica, ramulifera, 

 andicola). 



Lastly, the wood which forms the floating apparatus of the stems of certain 

 Leguminosae of the genera ^schynomene and Herminiera, which vegetate on the 

 surface of the water, may deserve mention. Their structure appears to be very 



1 Regnault, I.e. p. 118, pi. VI. 



^ Compare Brongniart, I.e. — Schleiden, Anat. d. Cacteen, I.e. (p. 156). 



K k 2 



