ANOMALOUS THICKENING IN DICOTTLEDONS AND GYMNOSPERMS. 571 



behind by that of the rest of the circumference, while the converse holds in the 

 production of secondary bast. While in other respects the wood inxn-eases as usual 

 among the woody Dicotyledons, and remains surrounded by a relatively narrow layer 

 of bast, it first appears depressed in the four longitudinal bands indicated, and is soon 

 , interrupted by furrows limited by flat lateral surfaces, which increase in depth as the 

 thickening proceeds : the wood thus becomes four-lobed in transverse section : 

 the groove is, however, exactly filled up by a bast-plate which runs out externally 

 into the original bast, so that the cylindrical or ribbed surface of the stem suffers 

 no important change of form through the unequal growth of the wood. The 

 cambial zone surrounds, on the one hand, the four prominent portions of the wood 

 as far as the margins of the bast-plates, and on the other hand, the outer surface of 

 the four bands of wood of which the growth is suppressed : it forms at both points 

 both wood and bast, but in unequal quantity as described. On the lateral surfaces 

 of the bast-plates, from the margins to the inner surfaces of the furrows, it is inter- 

 rupted ; as soon as the inflexion of the wood begins, the cells at the margins of the 

 grooves lose their cambial properties, and assume those of parenchyma of the 

 medullary rays. At each margin of each of the eight sections of the cambial zone 

 thus separated, the cells immediately adjoining it form henceforth only parenchyma 

 of medullary rays ; those of the margin of the projecting portions of wood form 

 especially cells of medullary rays of the xylem in centrifugal succession, those at the 

 base of the groove forming cells of the bast rays in centripetal succession. 



The lateral surfaces of each bast-plate are therefore limited by one pluri- or 

 multi-seriate medullary ray, which may be divided into one radial portion belonging 

 to the bast-plate, which grows centripetally, and one belonging to the contiguous 

 projection of wood, which grows in the main centrifugally. As a result of the 

 non-uniform progression of its growth, a continuous displacement goes on between 

 the two radial portions, in other words, between the lateral faces of the bast-plate 

 and the adjoining ones of the woody projections ; the two faces are not grown together 

 one with another; in transverse sections, even of fresh internodes, a slit-like space 

 often appears between the two ; the bast-plate is in close tissue-connection with its 

 surroundings only at the outer and inner sides. But, as has been already said, there 

 is also a slow growth of wood from the portions of cambium at the base of the 

 groove, and accordingly as this goes on, there succeeds a firm coalescence (and 

 usually lignification) of the radial portions of the medullary ray, as far outwards as 

 the formation of wood extends. 



The finer structure of the wood shows no specially remarkable peculiarities in 

 those species, which have been more exactly studied. Their bundles consist, e. g. in 

 Bignonia (Anisostichus Bur.) capreolata\ of wide-pitte^i vessels, narrow spirally 

 thickened pitted vessels, and vessel-like tracheides, wood-fibres, paratracheal bundle- 

 parenchyma, and intermediate fibres : they are traversed by numerous one- or few- 

 layered medullary rays, to which may be added the four at the limiting faces of the 

 four portions of wood which are superseded. These latter portions, besides their 

 limited development, are distinguished from the rest of the wood in the species 

 named by their denser structure, resulting from the absence or paucity of wide pitted 



' Compare Sanio, Botan. Zeitg. 1863, p. 407. 



