18 STIGMARIA FICOIDES. 



Hitherto I have failed to discover any specimen showing the exact relations of 

 the exterior of this cambiform zone to the innermost one of the true cortex ; hence 

 I am unable to say whether or not the former adds in any way to the growth of the 

 latter, or, in other words, whether any proper phloem zone exists in a Stigmarian 

 bark. The probability that something of the kind will be found is suggested by 

 the fact that we appear to have a true phloem element in the vascular bundles of 

 the rootlets, as will be shown on a later page. In Plate VII, fig. 10, we discover 

 at d d some groups of what appear to be irregular cells intermingled with the 

 cambiform cells, and which appear to be inward extensions of a more external 

 parenchymatous layer, the exact nature and relations of which I do not yet fully 

 understand. 



The Cortex. 



This structure, so far as I understand it, consists of three zones, which pass 

 more or less gradually into one another. In the cortex of my youngest specimen 

 (Plate IX, fig. 18) only two zones are seen, d and d', as shown in Plate X, fig. 21, 

 enlarged 18 diameters. The outermost of these, d, is parenchymatous, and 

 the cells are without any special arrangement; in the inner zone, d', the cells 

 are arranged in radial parallel lines ; the transition from the one to the other 

 being rather abrupt. The thickness of the combined layers is about ^ ( = *05) of 

 an inch, that of the inner one, b, being about -£g to -^ of an inch. 



Having no longitudinal section of the above specimen, we learn from it 

 nothing of the relations of the two layers. But when we turn to roots of larger 

 dimensions and older growth, such sections throw some light upon the matter, 

 though their structure is more complicated. The outermost cortical layer (Plate 

 VIII, fig. 15, d, and Plate VI, fig. 9, d) is always, as in the above-mentioned fig. 

 21, a simple parenchyma. But, though the lengths and breadths of the cells are 

 about equal in all directions, the thickness of the layer as a whole varies in different 

 specimens. The sizes of the cells vary in the same specimen, and still more 

 in different specimens, the latter differences being due to age. 



In the very young example, Plate X, fig. 21, the cells average from ±^ ('0025) to 

 -g-^o (-00125), the mean thickness of this layer of parenchyma being about -^ (-016) 

 of an inch. In the older specimen represented in Plate VI, fig. 9, the corresponding 

 layer has a maximum thickness of about y^ ('1) of an inch, and its cells vary from 

 YJq ('006) to 3-^7 (-003) in diameter. Hence, whilst the outermost bark in the 

 older of these two specimens, as contrasted with the younger one, has increased in 

 breadth six times, its component cells have less than three times the diameter of 

 the younger ones. Such being the case these cells must have more than doubled 



