ORGANIZATION OF. THE FOSSIL PLANTS OF THE COAL-MEASURES. 713 
the leaf of a Cycad at once snggests itself. Longitudinal sections justify the com- 
parison, for they prove that the small xylem-elements are spiral trachee, which thus 
constitute the protoxylem or.first-formed elements of the primary wood (see Plate 22, 
fig. 4, which represents a radial section, passing through a primary xylem-strand, 
bordering on the pith). It will be seen that the tracheides to the exterior of the 
protoxylem are.scalariform, while those on its inner side are pitted. There is a sharp 
distinction between the primary xylem and the tracheides of the secondary wood. 
The structure described remains constant throughout the stem. The position of the 
protoxylem is maintained, as the bundle passes out from the pith (see Plate 23, fig. 9) 
and remains the same after it has reached the pericyle (see Plate 22, figs. 5 and 6). 
It cannot be too strongly emphasized, that the protoxylem does not lie on the limit 
of the primary and secondary wood, but is placed in the interior of the primary strand 
itself, so that the development of the primary wood must have been partly centripetal 
and partly centrifugal. This fact is well illustrated by fig. 6. The leaf-trace shown 
in that figure has reached « point on its outward course where secondary wood is no 
longer formed, yet the position of the protoxylem in the interior of the ligneous 
strand is quite obvious. That the centrifugal wood to the outside of the protoxylem 
is not secondary but primary, is proved, both by the irregularity of its arrangement, 
as shown in transverse section (see figs. 2, 3, 5, 6, and. 8), and also by the character of 
the markings’ on its walls (see fig. 4). The limit between the centrifugal primary 
wood and the true secondary wood with radially arranged elements is perfectly sharp 
(see figs. 2, 8, 4, 5, and 8). In many cases the two are actually separated by 
parenchymatous cells. 
We are convinced that the same interpretation holds good for the foliar bundles 
of Cycadem, and that here also the centrifugal part of the wood must be regarded as 
a primary structure, though in certain casés it may receive subsequent additions 
from a cambial layer. ; 
We regard then the structure of the vascular bundles in the stem of Lyginodendron 
as idéntical with that of the foliar bundles of Cycadez. 
This type of structure has been called diploxylic, but this term is so used by MM. 
BertRanp and ReENavLt* as to imply that the centrifugal part of the wood is 
secondary. We may either coin a new term, and call bundles in which the protoxylem 
lies in the interior of the primary strand of wood mesomylic, or adopt the word 
mesarch, suggested by Count Sotms-Lausacu.t The stem-bundles then of Lygino- 
dendron, like the leaf-bundles of Cycadez, are mesarch or mesowylict in structure. 
* “Faisceaux foliaires des Cycadées actuelles,’ ‘ Archives Bot. du Nord de la France,’ 1886. 
t “Fossil Botany,” p. 257. The term mesarch, which has the advantage of being the shorter, implies 
that the development begins in the middle of the strand of wood. 
t This new term corresponds to the terms pertrylic and centroxylic used by M. Van Tizauem, the 
former term implying that the protoxylem is peripheral, the latter, that it is directed towards the centre 
of the stem. In mesowylic bundles the development of the primary wood is partly centripetal, partly 
centrifugal. 
MDCCCXCV,—B, 42 
