404 Scott. — On Peduncle of Cycadaceae. 
altogether towards the apex. Fig. i gives a fair idea of the 
extent of its development in the middle part of the peduncle. 
In this region almost every bundle shown in any transverse 
section possesses it, and often in considerable amount ; at 
least two dozen elements of centripetal xylem often occur 
in connexion with a single bundle (see Figs. 3 and 4, x' 1 ). 
Probably every vascular bundle in the peduncle has centri- 
petal xylem in some part of its course. 
The nature of these internal lignified elements is shown 
in longitudinal sections, which prove that they are, as a rule, 
scalariform tracheides, like those which form the greater part 
of the centrifugal wood (see Figs. 5 and 7). The centripetal 
tracheides are, on the whole, shorter than the centrifugal. 
The position of the centripetal xylem is such that its 
elements are always placed within the circummedullary zone ; 
in some cases they lie at its inner margin, separated by 
a considerable interval from the spiral tracheides (Fig. 5) ; in 
other cases they are situated further to the exterior, and 
are sometimes almost in contact with the protoxylem (see 
Figs. 3 and 7). 
The extent of the centripetal xylem and the arrangement 
of its elements in the peduncle of St anger ia is precisely com- 
parable to that found in certain foliar organs of Cycadaceae, 
as, for example, in the stalk of the carpel of Cycas revoluta 
and other species of Cycas and of Encephalartos. The com- 
parison of the structures, as shown, for example, in Fig. 3, 
from the peduncle of Stangeria , and Fig. 6, from the carpel 
of Cycas, can leave no doubt that the internal tracheides in 
the former are really homologous with the well-known centri- 
petal xylem of the foliar bundles of Cycadaceae. 
The centripetal elements which approach the protoxylem 
are, on the whole, smaller than those remote from it, just as 
in the case of the centrifugal wood (Fig. 4). The protoxylem- 
elements of the peduncle are usually in contact with the 
centrifugal part of the wood, whereas in the foliar organs 
of Cycads the reverse is more commonly the case. There 
are, however, so many exceptions to the latter rule, especially 
