6o2 



SECONDARV CHANGES. 



segments of wood undergo considerable increase at their outer side, and are extended in a 

 fan-like manner so as to grow into the bast-plates, and since fan-like excrescences grow 

 out also from the lateral margins of the main segments of wood. Each excrescence 

 of wood starts from a corresponding segment of cambium, that of the short superseded 

 segments undoubtedly from the original segments of cambium, that of the lateral margins 

 of the large segments from secondary cambium ; a further though weak production 

 of bast by the segment of cambium corresponds to each excrescence of wood. The 

 process in A. capreolata has not, as it appears from other writers, been observed much 

 further than here described and illustrated. But in other Bignoniae^ the splitting of the 

 w-ood in the first instance goes further ; it is broken up successively into numerous 

 segments corresponding in general to its dichotomies : these are separated by radial 

 bands of parenchyma, and surround a central portion, in the transverse section of which 

 are seen numerous strands of wood of various form. It is uncertain how far the latter 

 are of secondary origin, or are thrown off from the old wood by corresponding growth 

 of parenchyma. That the latter is the mode of origin of some of them, and is provided 



for to a certain extent from the first, is shown to be 

 probable by the islands of parenchyma, which lie near 

 to the medullary sheath in the young still undivided 

 xylem of Melloa populifolia (Fig. 226). In the old 

 stem the xylem is cut up throughout by broad bands 

 of parenchyma and bast, and especially a number of 

 central bundles are separated from the peripheral 

 ones. 



The separate segments of wood increase in 

 thickness on all sides, either by segments of the 

 original cambium surrounding them, or by secondary 

 cambium ; and it can hardly be doubted that the latter 

 always forms new bast elements on the side op- 

 posite the wood. How great a confusion of seg- 

 ments of wood, and bands separatmg them, may 

 arise by the growth described, and by successive 

 splitting, is shown by Criiger's not very accurate 

 figures of Bignonia Unguis'^, and by Fig. 238, re- 

 produced from Schleiden's Grundziige, which Criiger 

 regards as the transverse section of an old stem of 

 Bignonia, but Schleiden as a Bauhinia. 



FIc;. 237.— Anisosiiclius caprculan (Uijfiioma 

 I„). *rransverse section through an old stem. 

 Natural size. Compare tlie young stem, Vig. ^24, 

 p. 570. The four projecting segments nf the xylem 

 are completely split apart from one another to the 

 middle, by extension of the pith and of the bast- 

 plates. Three of the small superseded segments of 

 wood lie also separately in the surrounding tissue, 

 and are extended in a fan-like manner at the peri- 

 pheral margin. Two of the four segments of wood 

 are further radially split, in the case of the lower 

 one to the left, as far as the pith. From the margins 

 of the segments of wood numerous excrescences 

 project in a fan-like shape into the plates of bast 

 and parenchyma. Bast and parenchyma are shaded, 

 the wood is left white, with the exception of an in. 

 dication of annual rings and medullary rays. Ex- 

 ternally the cortex is surrounded by fissured bark, 

 which is drawn in black. 



Similar phenomena of splitting and of in- 

 dependent further growth within the simulta- 

 neously growing parenchyma are shown by the originally more or less lobed 

 xylem of climbing Malpighiaceae^. Special generic peculiarities cannot at present 

 be definitely laid down for this family. The splitting of the medullary sheath and of 

 the rest of the wood by the growth of the masses of parenchyma, the independent 

 growth in thickness of the separate segments of wood, which have no special 

 pith of their own, have been already clearly described by Jussieu in this group, if 

 it be overlooked that he often calls the growing intercalary parenchyma cortical 

 tissue intruding (from the outside). In a living branch of Stigmaphyllum ciliatum 

 i^T^^ thick I have myself been able directly to follow the dilatation of the pith and of 



' Compare Criiger, Botan. Zeitg. 1850, I. Taf. II. 



2 Botan. Zeitg. 1850, Taf. II. 



^ Compare A. de Jussieu, /. c. ; Criiger, /. <-. ; and alwve, p. 577. 



