264 
Robertson. — Some Points in the 
in the family, at least strengthens the presumption that a tendency to the 
production of centripetal xylem is a hereditary character of the Taxoideae. 
So I am inclined to believe that the occurrence of centripetal xylem in the 
cladodes of Phyllocladits is quite possibly due to an inherited tendency 
derived by both Phyllocladus and the Taxoideae from some common 
ancestor, and apt to reappear when a special combination of circumstances 
renders it appropriate. In the case of Phyllocladus the use of the centri- 
petal xylem is not far to seek. As shown in Fig. 2 the cladode is flattened 
parallel to a plane which would symmetrically divide the lateral bundles, 
i. e. the latter occupy a position at right angles to that which they would 
take up if the cladode was a leaf. An extension of the xylem in the plane 
of flattening of the organ is brought about in leaves by wings of transfusion 
tissue lateral to the bundle (e. g. Podocarpus ), whereas in Phyllocladus 
an extension of the wood in a corresponding plane necessarily brings 
centripetal xylem into existence. If we suppose that the centripetal 
xylem is an ancestral character, one alternative is to imagine that at 
that remote period when the cladodes were first being evolved the axis 
and leaves still possessed internal xylem, so that the cladode naturally 
possessed it as well ; it must then have gradually disappeared from the axis 
and leaves, while it was retained in the cladode owing to its usefulness. On 
the other hand, it is, I think, open to us to suppose that the internal xylem 
was completely lost in stem and leaves before the flattened leaf-like 
branches were ever developed, but that when the cladode was evolved the 
need of lateral transmission of water awoke, as it were, a forgotten memory, 
and the plant once again took up the formation of centripetal xylem. 
4. Taxinean sculpturing occurs in the tr ache ids. In Phyllocladus 
alpinus tracheids with the characteristic Taxinean markings (a combination 
of bordered pits and spiral or scalariform thickenings) occur in the xylem. 
Tracheids of this kind, though by no means confined to the Taxoideae, are 
sufficiently characteristic of it to make their appearance in Phyllocladus 
distinctly suggestive of affinity. 
V. Summary. 
The main result of this investigation is to confirm the view of the 
systematists that Phyllocladus occupies an intermediate position between 
Podocarpoideae and Taxoideae, but with greater affinity for the former. The 
existence of winged multi-nucleate pollen-grains ratifies the Podocarpoid 
affinity, while the presence of centripetal xylem in the cladode is regarded 
as a ‘ harking-back ’ to a hypothetical common ancestor of Taxoideae and 
Phyllocladoideae which possessed bundles of this type. 
