189 
Xylems in the Petioles of Cycads. 
exception of the protoxylem, perhaps entirely secondary, for it is not easy 
to see where to draw a limit in rows of cells which look so uniform. On 
the other hand, it would be easy to show that the centripetal xylem is 
a primary structure, the result of the division of the meridesmogen strand ; 
there is no sign of cambium activity, the cells are distributed in their proper 
place a long time before they are gradually lignified. In fact, the primary 
nature of the centripetal xylem is now generally admitted. 
But, if this is the case, it is difficult to regard a primary structure as 
the continuation of a secondary one. The continuity theory breaks down 
here altogether : to be in succession or in contact is a different thing from 
being continuous; to be in succession may mean only juxtaposition ; to be 
continuous implies, moreover, community of origin or derivation. Such 
structures could not therefore be called homologous, and still less identical 
with each other. It is much like the Poroxylon described by Bertrand . 1 
The difference in the nature of the xylem cells could also be quoted in 
favour of this statement ; but this proof is of little value, as the nature of the 
centripetal xylem varies so much and so rapidly with the distance from the 
protoxylem, giving in succession spirally thickened, reticulate, annular, 
scalariform, pitted, multiseriate pitted tracheides (Fig. 11). 
There is a remarkable resemblance between the vascular tissue of the 
Cycad petiole and that of a leaf of Cordaites , as described by Dr. Marie 
Stopes . 2 The cells forming the inner sheath are very like the reduced 
centrifugal xylem of Cycads both in longitudinal and transverse sections, 
the only visible difference being the position. In the Cordaites described 
they are in close relation to the bundle-sheath, but in many leaves of this 
genus a large amount of phloem separates the centrifugal xylem from the 
bundle-sheath. There seems to be no adequate reason for regarding the 
inner sheath as other than the centrifugal xylem. 
Dr. Carano insists strongly on the fact that the centrifugal xylem is in 
continuation with the centripetal xylem or even the protoxylem. But this 
relation is more apparent than real, if we admit that the centrifugal xylem 
and centripetal xylem are juxtaposed rather than continuous. It is only at 
the very base that the xylems join hands and that the protoxylem becomes 
common to both. 
The facts seem to bring out the independence of the xylems ; they are 
independent in origin and nature but are juxtaposed to carry out their 
functions ; in fact, in places they are brought into communication with one 
another by transfusion cells, or by the laterally stretched wings of the centri- 
petal xylem. They are continuous physiologically but not morphologically. 
This interpretation has much in common with the view supported by 
Bertrand and Renault. The centrifugal xylem is the continuation of the 
1 C. Eg. Bertrand et B. Renault (’86). Cf. alsoD. H. Scott, Studies in Fossil Botany, 1909. 
2 Marie C. Stopes (’ 03 ). 
