CORTEX. 21 



been much greater, since the specimen has evidently been much compressed ; the 

 dark disorganised bands, e (fig. 26), having normally been composed of the same 

 tissues as the intermediate bands, e, in which the radial lines of cells are less 

 disturbed. 



The preceding facts make it evident that the prosenchymatous zone e has 

 originated in the centrifugal meristemic action of the zone d\ a conclusion which 

 agrees with my previous determination as to the centrifugal development of a 

 similar prosenchyma in the stems of Sigillaria and Lepidodendron} 



But a second physiological question is less easily answered. We have already 

 seen that growth is accompanied, not only by a general increase in size and by a 

 dilatation of the individual cells of the outer zone, d, of the bark, but by a considerable 

 increase in their number. Whence have these new cells been derived ? I have 

 never observed any signs of meristemic action amongst the cells actually composing 

 this layer, yet they must have been multiplied somewhere by such action. I am 

 therefore disposed to conclude that the meristem zone, d', of Plate VIII, fig. 23, has 

 added centripetally to the outer parenchyma, d. If so, the true position of the bark- 

 cambium must have been somewhere about the centre of the zone d', where the tan- 

 gential septa were most closely approximated, and where the secondary horizontal 

 fissions, seen in the tangential section (Plate VIII, fig. 24), were being most actively 

 produced. This view, if correct, explains the disturbed arrangement at the inner 

 part of the zone d in fig. 23, where the parenchymatous cells were being liberated 

 from the more internal radial lines in which they had undergone their meristemic 

 development, but had not yet assumed the characteristic form seen in the layer d 

 of fig. 22. 



We have thus strong reasons for concluding that, in addition to a truecambial 

 layer which produced fresh zones of xylem, these Carboniferous Cryptogams also 

 possessed a bark-cambium which acted both centrifugally and centripetally, like 

 the phellogen of recent Exogens, only instead of producing true phellem externally 

 and phelloderm internally, this active zone produced parenchyma externally and 

 prosenchyma internally. What has already been described, however, shows that 

 the meristemic energy must chiefly have been expended on its inner side, since the 

 prosenchymatous layer evidently constituted by far the largest element of the 

 Stigmarian bark. 



In no case does this latter layer seem to have become a periderm. It is always, 

 in the specimens which I have obtained, enclosed within and protected by the 

 parenchymatous zone, d } of which the rootlets are partly an extension, and upon 

 which they are planted. Hence we may be assured that so long even as the mere 

 bases of these rootlets remained intact, the parenchymatous periderm remained 

 equally so. That such must have continued to be the case so long as the increase 

 1 See ' Memoir,' Part II, pp. 285, 286. 



4 



