132 WorsdelL— The Structure and Origin of the Cycadaceae. 
reinforced by an ‘ accessory transfusion-tissue,’ consisting of the central 
elongated cells of the mesophyll which have become transformed into 
tracheides, and which extend from the ordinary transfusion-tissue to the 
margin of the leaf. 
In all genera except Cycas and Stangeria a number of equal, parallel 
bundles traverses the lamina. 
The vascular bundles of the petiole of the leaf are typically arranged 
in the form of an S2 as seen in transverse section, with the xylems directed 
inwards or towards the ventral surface of the leaf. There are, however, 
various complex modifications of this arrangement of which the extreme 
case is attained in Encephalartos , in whose petiole the bundles are extremely 
numerous, and are orientated in every conceivable direction. The petiolar 
bundles of Cycads possess the same mesarch structure as those of the 
lamina, but the centripetal xylem is very great in amount, and is thus 
strongly contrasted with the very small development of the primary 
centrifugal xylem. 
As regards the vascular anatomy of the cone, the general feature is the 
possession of a single cylinder of separate bundles, each bundle being 
collateral in structure with a large amount of secondary superadded to the 
primary tissue. In certain genera ( Stangeria , Bozvenia , Ceratozamia) the 
endarch structure of the bundles at the very base of the peduncle is super- 
seded throughout the rest of the length of that organ by the mesarch 
character, owing to the presence of scattered primary centripetal tracheides, 
to which is sometimes attached a protoxylem-group distinct from that of 
the centrifugal part of the xylem. 
The sporophylls are spirally arranged on the cone and are short, highly- 
modified structures, each with a very thick shield-like terminal portion and 
a narrow stalk. The very numerous male sporangia are scattered in sori 
over the lower surface of the sporophyll ; the female sporangia are two in 
number, seated likewise on the lower surface of the organ, but on the 
adaxial side of the peltate portion. Two bundles supply each sporophyll as 
a rule; these branch so as to forma transverse row of strands. The bundles 
of the sporophyll which are situated below the level of insertion of the 
sporangia are characterized by the large amount of centrifugal, as compared 
with the centripetal, xylem which they possess as a result of their sporan- 
gium-supplying function. In the peltate portion of the sporophyll these 
relations are reversed. 
Owing to the fact that they are not compactly grouped together in 
a cone but are borne loosely on the vegetative axis of the plant, the female 
sporophylls or carpels of Cycas are more or less leaf-like in character, and 
possess rudimentary pinnae in their upper sterile portion. The ovules are 
usually about six in number ( Cycas Normanbyana has two), and are borne 
on the sporophyll in the lateral position of pinnae. 
