ANOMALOUS THICKENING IN DICOTYLEDONS AND GYMNOSPERMS. 589 



very narrow, and at most only some few cells wide, the sclerenchymatous ring appears at 

 its outer margin, and then within it the beginnings of bundles and medullary rays, and of 

 the cambial zone between wood and bast. The outer ring of sclerenchyma is pushed 

 outwards by the growth in thickness derived from the latter. The often-mentioned 

 thick-walled interruptions of the bast zone can hardly be derived otherwise than by the 



ultimate conversion of cells of the cambium into thick-walled permanent elements. 



« 



(2) In the large majority of cases, the bast-zone itself is the place of origin of the 

 successive rings of thickening ; that is, either its outermost (primary) region, as e. g. 

 in Phytolacca, or the outer, older zones of the secondary bast : thus, e. g. in Wistaria 

 chinensis, in stems of Lianes passing under the name Bauhinia, and others cor- 

 responding to the Leguminous plant called by Criiger Rhynchosia phaseoloides : 

 further, in the Liane above-mentioned, represenled by Miiller (/. c. Fig, 6) as Securi- 

 daca, and (Fig. 7) as Tontelea (.?). The same holds for Gnetum, at least for that form 

 represented in Fig. 233, for Phytocrene, Fig. 227, according to Eichler for Dolio- 

 carpus Rolandri, and also for those cases to be mentioned directly, which have not 

 yet been carefully investigated in connection with the question under discussion. 



The examples cited show that these phenomena belong to very different families, 

 and occur in plants and parts of plants adapted to very different modes of life. In 

 the first place they appear with a remarkable frequency in stems of Lianes, and more 

 especially at least in twining stems, not in those which climb with tendrils, as in 

 Wistaria, IMucuna sp. (F. Miiller), and other Leguminosae already quoted ; in 

 Comesperma and Securidaca volubilis from the Polygalaceae ; in Tontelea sp. 

 (Hippocrateaceee) ; Cocculus, species of Cissampelos and other Menispermaceae'; 

 Doliocarpus and other Dilleniacese -; Phytocrene, Gnetum; rarely also in Aristo- 

 lochias, as shown by Fig. 219, taken from Schleiden's Grundziige, see p. 550. 

 Lianes with tendrils are not however sharply excluded from such structure as that 

 under discussion, as shown by old stems of the Sapindaceous genus Thinouia (p. 583), 

 and the Bignoniaceous plants Haplolophium and Glaziovia (p. 575). Also the solitary 

 cortical bundles, which A. de Jussieu describes in Anisostichus capreolata, may belong 

 to this category. On the other hand, it is to be noted that stems of Lianes, which 

 are closely related to the species here mentioned, such as Menispermum canadense, 

 and most climbing Aristolochias have no renewed zones. And I may further mention 

 the appearance of the latter not only in the stem of the non-climbing Menispermaceous 

 plant Cocculus laurifolius, but also in others, as far removed as possible from being 

 climbing forms, such as the Avicennias mentioned above, Phytolacca, and Cycadese, to 

 which may be further added IMaerua uniflora (of the Capparidaceai) from East Africa, 

 Mith a stem structure Hke that of the above-mentioned Tontelea. In some Lianes, 

 Wistaria, Menispermese, and Securidaca volubilis (Criiger), the structure and course of 

 growth described extends also to the roots ; most of them have not been investi- 

 gated on this point. On the other hand, it is found, as far as is known, as a 

 general family-character in the roots of species of Phytolacca, Chenopodiaceae, 

 Amarantacece, &c.. even if the stem shows another character. Comp. Sect. 192. 



We shall return in Sect. 194 to the phenomena of growth of the roots of 

 Convolvulaceae, which also belong to some extent to this category. 



* Eichler, I.e., and in Flora brasiliensis, Fasc. 38, p. 207, Taf. 50, 51. 

 - Criiger, I.e. — Eichler, in Flora brasiliensis, Fasc. 31, p. 116, 



