922 PROFESSOR W. C. WILLIAMSON AND DR. D. H. SCOTT ON THE 
absence of developmental data, such delimitation of the tissues is necessarily 
arbitrary. 
The thin-walled zone is very sharply marked off from the rather sclerotic cortex. 
The latter consists of somewhat elongated cells, which become narrower and more 
sclerenchymatous as the exterior is approached. The structure of the epidermis is 
not clear in any of our specimens.”* 
The vascular system is strictly cauline; it passes through the nodes without any 
appreciable change of structure. A single bundle entered each leaf; from the analogy 
of other species of Sphenophyllum, it may be presumed that the foliar bundles were 
given off from the angles of the central strand. The number of foliar bundles given 
off at each node cannot have been less than eighteen.t It is possible that these may 
have arisen from the subdivision, within the cortex, of a smaller number, but as 
regards this species, such a conclusion is merely conjectural. 
For the mode of branching of the stem, we have only the evidence of one pre- 
paration.t In this case a single branch evidently arose at the node ; its vascular 
cylinder was to all appearance given off from one of the angles of the stele of the 
main stem. 
Secondary Changes. 
In S. plurifoliatum, as in other species of the genus, a large amount of secondary 
tissue, both wood and bast, was formed in the stem as growth proceeded. We have 
specimens with the secondary wood of every thickness, from a single layer of elements 
up to thirty-seven such layers. Some of the stages have already been figured.§ 
Others are illustrated in the photographs and figures accompanying the present: 
paper. There is no break in the series, and, except for the changes involved in the 
secondary growth itself, the structure is the same in al]. Hence there is no doubt 
that we are dealing with successive stages in the development of one and the same 
plant. The great extent of the secondary cortical tissues is a characteristic feature 
of the genus. 
We will begin our description with the secondary wood. 
The radial seriation of the elements is in most cases remarkably regular (see the 
photographs on Plate 75; also Plate 83, figs. 40-43). In one large specimen, for 
example, the rows are quite continuous throughout the secondary wood, which here 
attains a thickness of twenty-four elements (photograph 21). The same is the case 
in the specimen, a portion of which is shown, in transverse section, in photograph 22. 
Here the maximum thickness of the wood is thirty-seven elements. 
* For the details of primary structure, just described, reference must be made to the Memoirs of 
WILLIAMSON above cited. 
+ See Wittiamson, loc. cit., Part IX., Plate 21, fig. 28 (C.N. 908). 
$ C.N. 908, see figure just cited. 
§ See Witniamson, loc. cit., especially Part. V. 
