4 8 
TV. Bancroft. 
cylinder, the edges of which become incurved to unite with two 
of the internal vascular bands; a connection between these and the 
normal vascular ring is thus maintained. A similar behaviour, it 
will be remembered, was noted in some specimens of Lyginopteris 
oldhamia, the anomalous vascular bands in this type being connected 
at the leaf-trace gaps with the normal vascular structures. The 
conditions in Ptychoxylou are more complicated however, for there 
are often two or three concentric systems of internal xylem and 
phloem, joining the external cylinder at different levels. Centripetal 
primary wood seems to be absent in the stem of Ptyclioxylon, 
though the leaf-traces appear to have the same structure as those 
of Lyginopteris. 
Dr. Scott, in agreement with Prof. Seward’s views (14, p. 490; 
cf. 15), suggests that the Cycadoxyleae “ may have been derived 
from some form resembling Lyginodendron oldhamium from which 
they have deviated in two principal respects.” Firstly, they have 
gradually lost the primary centripetal wood of the stem bundles, 
though apparently retaining it in their leaves. “ This change is in 
a Cycadean direction, for the recent Cycads (as well as the 
Mesozoic Bennettiteae) have wholly lost the centripetal primary 
wood in their vegetative stems, while retaining it in their leaves 
and occasionally in the peduncles of their cones” (14, p. 491). 
Secondly, the Cycadoxyleae became modified by the formation of 
inverted medullary wood and bast, a constant and marked feature, 
foreshadowed as an individual and unimportant peculiarity in 
Lyginopteris. In this also there is a certain likeness to some genera 
of Cycadaceae, for these are characterised by the formation either 
throughout the stem or locally, of extra wood and phloem, though 
in the Cycads these anomalous vascular structures are cortical and 
not medullary, as in the Cycadoxyleae. 
Reviewing the main points of Cycadean anatomy—(1) the 
retention of mesarch structure in petioles and peduncles of cones; 
(2) the parenchymatous nature of the wood ; (3) the multiseriate 
pitting of the tracheides; (4) in Bowenia, Cycas, Encephalartos and 
Macrozamia, accessory zones or groups of xylem and phloem in the 
vegetative stem, with the joining of cambiums through leaf-gaps 
and the consequent formation of concentric bundles—it is seen that 
all these features are foreshadowed amongst fossils of the 
Lyginopteridean alliance ; so that there is much support for Scott’s 
view that the Cycads sprang from such a type as Lyginopteris 
itself. The Cycadoxyleae cannot be considered on the main line 
