CORTEX. 19 



their number. We thus learn that cell -growth in these primseval trees seems to 

 have obeyed the same laws as regulate that growth in plants now living. 1 



The second layer of the outermost Stigmarian bark is one of some interest, both 

 morphologically and physiologically. In my ' Memoir,' Part IX, p. 353, 1 called 

 attention to the occurrence, in the Oldham calcareous nodules, of fragments of a 

 singular form of bark, which I referred to Sigillaria and Stigmaria. I have now 

 no doubt respecting the accuracy of this reference, and that the bark is identical 

 with that now under our notice. 



To understand its true nature we must examine this tissue alike in transverse, 

 radial, and tangential sections. Plate X, fig. 21, represents, as already observed, a 

 small portion of the outer bark of the very young specimen, Plate IX, fig. 18, enlarged 

 18 diameters; in this figure the layer now under consideration (fig. 21, d') appears 

 as a series of cells, differing little from those of the investing parenchyma, d, except 

 in their more uniform size, and their regular arrangement in parallel, radial rows. 



In Plate VIII, fig. 22, we have a transverse section of the corresponding 

 portion of the specimen Plate VIII, fig. 15, also enlarged 18 diameters. In this 

 section the layer d retains its parenchymatous form, but d', whilst still consisting 

 of radial lines of cells separated by tangential septaa, not only has those septee of 

 very unequal lengths, but each radial group is circumscribed by a strongly-marked 

 boundary line, d", separating it from the contiguous parallel groups. In Plate VIII, 

 fig. 23, we have a similar section to fig. 22, also enlarged 18 diameters, but taken 

 from a yet older root. We now find the layer d' of the preceding section has 

 become more complicated. The rows of tangentially divided cells have lengthened 

 radially, and though tangential divisions still predominate, even at the more 

 external part of the layer d\ we now find some radial divisions introduced amongst 

 the tangential septa of many of the groups. In addition, many of the innermost 

 cells of the parenchyma, d, contiguous to the outer ends of these groups, exhibit 

 a peculiarly disturbed arrangement that does not appear in fig. 22. 



In Plate VI, figs. 9, d', and 45, d', we again see the layer just described, but 

 now in radial vertical sections. The groups of cells are again disposed radially, 

 only the parallel tangential septa are now elongated vertically, instead of tangentially 

 as in transverse sections. The radial boundary lines d" of fig. 23 are again seen 

 in d" of this and similar sections. In my ' Memoir ' (IX, pp. 354, 355) I described 

 the cells thus separated from one another by tangential septa as " tabular cells 



1 " An increase in the average breadth of the individual cell no doubt takes place, judging from 

 estimates. It appears to rise rapidly to an approximately constant value, and then to maintain this 

 during succeeding divisions, so that cells of the same layer in a stem a foot thick are no broader than 

 in one as thick as one's finger, though they are of course more numerous in a corresponding degree. 

 The final, constant average dimensions are relatively little in excess of those existing originally at the 

 beginning of the growth in thickness ; they may amount to scarcely more than two or three times the 

 latter." (De Bary, ' Comparative Anatomy of Phanerogams and Ferns,' English translation, p. 538.) 



