Jeffrey. — On the Structure of the Leaf in Cretaceous Pines. 2 1 7 
evolution the centripetal wood was the first archaic feature to become 
blotted out. Its disappearance was complete and occurred at a compara- 
tively early stage of Coniferous history. At a later stage the inner trans- 
fusion sheath followed the centripetal wood into oblivion, although even in 
modern Conifers traces of it may still be found in the region of the proto- 
xylem. As the result of the disappearance of the centripetal wood and the 
inner transfusional sheath, the centrifugal wood became ultimately directly 
continuous with the jacket of ordinary transfusion tissue, the outer trans- 
fusion sheath of the more ancient Abietineae. The outer or ‘ peridesmic ’ 
transfusion zone in the course of these changes became more and more 
degenerate and intermingled with parenchyma cells, and came to possess 
the structure shown in the vegetative leaves of modern Pines. The outer 
sheath is also persistent to some extent in the leaves of the Araucarineae, 
but even the older known representatives of this family show no indication 
of the existence of true centripetal wood or of an inner transfusion sheath. 
The new data supplied by present investigation and by other recent 
researches on living and extinct Coniferales seem to strengthen the 
hypothesis put forward over two years ago by the author, to the effect that 
the Abietineae are a very old, if not the oldest, family of the Coniferales. 
It specially stands out in connexion with these results, that the Abietineae 
must be considered more primitive than the Araucarineae, which are 
generally regarded as the ancestral Conifers. The arguments for this view 
may be summarized as follows : — 
1. The possession on the part of the Abietineae of marked vestiges of 
a double leaf-trace, such as is almost universally characteristic of the older 
Gymnosperms. 
2. The presence of true centripetal wood in the genus Prepinus , which 
may be regarded with a strong degree of probability as the ancestor of the 
living genus Pinus. 
3. The marked and detailed resemblance of the foliar bundle in 
Prepinus , not only in the presence of centripetal wood, but also in the 
complex double sheath of transfusion tissue, to certain Cordaites . 
4. The persistence of the double transfusionary sheath in the true 
Pines of the Middle Cretaceous, although no similar structures have been 
found in numerous representatives of contemporary Araucarineae. 
5. The wound-reactions of the older Cretaceous Araucarineae, referred 
by the author to the new subfamilies Brachyphylloideae and Araucario- 
pityoideae, indicate a derivation of the ancestral Araucarians from an 
Abietineous stock. 
6. The pitting of the older Araucarineae, which still survived in the 
Middle Cretaceous, showed a marked deviation from that found in Agathis 
and Araucaria , and a transition towards the type of pitting found in the 
Abietineae, while the oldest structurally known type of the Abietineae, 
Q 
