SECONDARY INCREASE IN THICKNESS OF STEMS AND ROOTS. 1 37 



direction. These lobes, separated by radial lamellae of tissue resembling cortex, 

 may again be divided in the same manner into smaller lobes; as occurs in some 

 Malpighiaceae like Heteropterys and Bannisteria. In some species of Bignonia the wood, 

 as seen in transverse section, forms a four-armed cross, the arms of which are 

 separated by very broad soft masses of tissue resembling medullary rays, which pass 

 outwards into the cortex surrounding the stem (see Fig. 107). In this order Tecoma 

 radicans has also the peculiarity, noticed by Sanio, that a new ring of wood is 

 formed in the pith, altogether separated from the medullary sheath of the ordinary 

 ring ; this new ring has on its inner side a secondary layer of phloem, and is increased 

 by cambium. 



(e) Some of the most peculiar modes in which the wood is formed are found in many 

 climbing Sapindaceae. The abnormalities here described are, in fact, met with chiefly 

 among lianes, i.e. woody cHmbing and twining plants growing in tropical forests; 

 although there are typical liane-stems which do not climb. In the abnormal Sapindaceae, 

 especially the genus Serjania, a transverse section shows three, four, five, or more portions 

 of wood completely separated from one another, of which the central one is usually 

 much the most strongly developed. Each of these masses of wood presents the 

 structure of a normal dicotyledonous stem ; medullary rays proceed from its central pith 

 towards its circumference ; each is surrounded by a narrow secondary cortex, by which 

 they are, however, united into a whole. Looked at from without, the more slender 

 outer portions of wood have the appearance of cushions, which project below out of 

 the central larger one, and coalesce again with it above. The separate masses, which 

 on a transverse section seems to be completely independent, are therefore only pro- 

 jections or outgrowths of the originally homogenous stem. According to Nageli this 

 abnormality is caused, at the very origin of the fibro-vascular bundles, by their not 

 being arranged in a ring, so that a single cambium-ring cannot unite them all ^. 



(f) In the cases hitherto described the deviation from the typical processes is 

 caused essentially by abnormalities in the origin of the cambium-ring, or by its sub- 

 sequent behaviour, or finally by repeated fresh formation of fibro-vascular bundles in a 

 secondary meristem. The case is diff'erent in Piperaceae, Begoniaceae, Nyctagineae, and 

 Amaranthaceae. An al?normality is here brought about at a very early period in the 

 growth of length of the stem, of such a nature that, in addition to the leaf-trace-bundles 

 which bend above into the leaves, vascular bundles also arise in the stem which do 

 not pass out into the leaves, i.e. 'cauline bundles.' On a transverse section even 

 of a very young intern ode a number of fibro-vascular bundles are seen dispersed 

 through the fundamental tissue ; and the alternative now presents itself whether the 

 leaf-trace- or the cauline bundles shall be united by cambium and hence become 

 thicker, or whether the formation of cambium shall be altogether suppressed. Sanio 

 states that the latter occurs among Piperaceae, in the genus Peperomia, where the leaf- 

 trace-bundles which constitute an outer circle are not united by a ring of cambium, 

 any more than the inner ones which do not bend out into the leaves. In Cha^vica, 

 on the contrary, this takes place in the outer ring of leaf-trace-bundles, while in the 

 pith which is enclosed by them the cauline bundles remain isolated and without any 

 secondary increase in thickness. The structure is the same in Begoniaceae. That of 

 Nyctagineae, on the contrary, to a certain extent varies from this; the thick leaf- 

 trace-bundles, ascending through the middle of the fundamental tissue, remain isolated, 

 while a wider outer ring of cauline bundles forms a ring of wood by the activity of its 

 cambium. 



Sect. 19. The Primary Meristem and the Apical Cell^. — At the growing 

 ends of shoots, leaves, and roots, the forms of tissue hitherto described do not 



^ Nageli (/. c.) describes the very complicated processes more fully. 



2 Nageli, Die neueren Algensysteme. Neuenburg 1847.— Cramer in Pflanzenphysiol. Unter- 

 suchungen, Heft III. p. 21, Zurich. — Pringsheim, Jahrb. fiir wissen. Bot. vol. III. p. 484. — Kny, 



