1 66 Holden . — Reduction and Reversion in the 
Cretaceous, remains have been discovered not only of Saliciphyllum 
and Popidophyllum , but also of at least twenty other Angiospermous 
genera. 1 Until more data are secured, then, it is impossible to prove 
anything regarding the phylogeny of the Angiosperms from palaeobotanical 
evidence. 
In view of these conflicting standpoints, it seems desirable that some- 
thing should be said regarding the anatomy of the Salicales. Before doing 
so it will be well to enumerate some of the principles upon which compara- 
tive anatomy rests at the present time. The fundamental one is Haeckel’s 
law of recapitulation. According to this we should look for primitive con- 
ditions in the seedling and the first annual ring, as applied to plants. 
Investigation has shown the soundness of this principle and has added 
other primitive regions, viz. the tissue about outgoing traces in both root 
and stem, the reproductive axis, the leaf petiole, &c. Further, experimental 
work has shown that reversion to a primitive condition often takes place 
after injury. It is from a study of these regions then, that phylogenetic 
deductions must be drawn. 
Turning now to the Salicales, Figs. 1,2, and 3(P1. XXjrepresent the wood 
of a typical poplar, Populus trichocarpa, in transverse, radial, and tangential 
planes respectively. From these it is evident that the rays are one cell in 
width, or uniseriate, and the parenchyma only at the end of the annual ring, 
or ‘ terminal \ Another diagnostic feature of the wood of the Salicales is 
the end-wall of the vessel, which has one large pore constituting the so-called 
porous perforation, as opposed to the scalariform perforation characteristic 
of Betula , for example. The type of fibres is obviously not primitive, and 
the type of vessel-end-wall is likewise high. 
Popidus tremidoides has normally the same wood structure as that of 
P . trichocarpa ; Figs. 4 and 5 show, however, transverse and tangential 
sections through the wood formed after injury. Here the rays are bi- and 
triseriate, and the parenchyma, instead of being terminal, is present also 
around the vessels. 
Seedlings of P. tremuloides were examined and show that, while the 
rays are always uniseriate, the parenchyma around the vessels, as well as in 
the terminal position, is common. This is especially true of the transition 
region between stem and root — in the hypocotylary stem. Above and below, 
the vasicentric parenchyma tends to die out. In the stem it is always 
present to a greater or less extent in the first annual ring, while in the root 
it persists fairly abundantly throughout. Fig. 6 represents a section through 
the root at some distance from the hypocotylary region and shows most of 
the vessels with one or two parenchyma cells next their tangential walls. 
Fig. 7 represents at the same magnification vessels with parenchyma cells, 
and Fig. 8 represents at a higher magnification another vessel with one cell. 
1 Fontaine : Potomac or Younger Mezozoic Flora. U.S. Geological Survey, vol. xv. 
