924 PROFESSOR W. C. WILLIAMSON AND DR. D. H. SCOTT ON THE 
as the remains of transverse walls in the trachese,* a view which he has elsewhere 
abandoned in favour of the true explanation.t 
The arrangement of the cells, two or three of which are often placed immediately 
above one another (see fig. 44) is quite inconsistent with the former view, which is also 
negatived by the fact that no such transverse markings are seen in a tangential section. 
It is important to notice that the radial cells in question do not, as a rule, form 
continuous radial series, and thus differ from the true medullary rays, such as are 
present in Sphenophyllum insigne. (Compare Plate 83, fig. 44, with Plate 84, fig. 49.) 
It must, however, be pointed out that the absence of continuous medullary rays is 
not constant even in S. plurifoliatum. In the outer layers of the secondary wood, 
continuous radial tracts of parenchyma, sometimes several cells in breadth, make their 
appearance. (See Plate 75, photograph 22.) 
The description given so far applies more especially to that part of the secondary 
wood which is formed between the angles of the primary xylem; we may term this part 
of the wood interfascicular, using this term for the tissue formed between the 
protoxylem-groups, in contradistinction to that which is formed opposite them ; for 
the latter the term fascicular wood will be used. 
The fascicular wood is sharply distinguished from the interfascicular, as seen in 
transverse sections. (See Plate 75, photographs 19, 20, and 21; Plate 83, fig. 43.) 
This is due in the first place to the much smaller dimensions of the fascicular trachez. 
The secondary wood is necessarily thinnest opposite the prominent angles of the 
primary xylem, for the general outline of the wood becomes circular as soon as 
secondary growth is established. The number, both of tangential and radial series is, 
however, usually greater in the fascicular region, so the reduced size of the elements 
here is easily explained. There is a general continuity of the concentric series of 
xylem-elements all round the stem ; the fascicular cells have simply undergone some 
additional subdivisions. The structure of the trachez is the same in both regions. 
The most important distinction consists in the presence of continuous medullary rays, 
from the first in the fascicular wood, while in the interfascicular region, they only 
appear, if at all, in the outer layers. The boundary between fascicular and inter- 
fascicular wood is fairly sharp in the inner secondary zones; the distinction, 
however tends to disappear in the outer part of the wood of very old stems (see 
photograph 21). 
That the whole zone of wood, outside the triquetrous strand, is really secondary, 
has been sufficiently shown by M. Van TizcHem.t In Sphenophyllum plurifoliatum, 
we are able to give the final proof, for in several of our specimens the cambium itself, 
* ‘Cours de Botanique Fossile,’ vol. 2, p. 99, 1882. 
+ ‘Ann. des. Sci, Nat., Bot.,’ Série 6, vol. 4, p. 297, Plate 8, figs. 2 and 4, 1877 (sic). ‘Cours de 
Botanique Fossile,’ vol. 4, p. 8, Plate C, fig. 4, 1885. 
tf “Sur quelques points de l’Anatomie des Cryptogames: Vasculaires.” ‘Bull. de la Soc. Bot. de 
France,’ vol. 30 (1883), p. 169. 
