SOME FEATURES OF THE ANATOMY OF THE FOLIAR 
BUNDLE’ 
EpmuNnpb W. SINNOTT 
(WITH PLATE XVII) 
Although the vascular system of the leaf in the Cycadaceae has 
for some time been recognized as a region where primitive struc- 
tures, notably centripetal wood, have persisted longer than any- 
where else in the plant, the general principle of the conservatism 
of the foliar organ has only recently been more widely extended 
and found to hold good for all the main groups of vascular plants: 
Lycopsida, Filicales, Gymnosperms, and Angiosperms. The posi- 
tion of the protoxylem in relation to the later formed elements of 
the wood is very constant in the large orders, but its importance 
in determining phylogenetic relationships seems often to have 
been overlooked. A brief summary of the structure of the leaf 
trace and foliar conducting system of all families of vascular plants, 
with special reference to the position of the protoxylem, and a 
more particular account of conditions in the living Cycadaceae, 
the only existing gymnosperms with centripetal wood, together 
with a discussion of the bearing of the results on our views as to the 
evolutionary history of the higher plants, is the purpose of the pres- 
ent paper. 
That the base of the leaf trace, since it is least subject to changes 
in external conditions, should retain ancestral structures longet 
than does any other portion of the foliar system seems logical, am 
this has been proven to be true in several groups of vascular plants. : 
It is therefore in this region that we should look for the primitive 
structure of the foliar bundle. 
In all the Lycopsida where the anatomy of the leaf is known 
(Equisetum, Lycopodium, Selaginella, Phylloglossum, Tmesipler® 
Psilotum, Isoetes, Lepidodendron, and Sigillaria), the vasculat 
strand at the base, and in the majority of instances throughout 
the lamina, of the leaf, is a single concentric strand, roughly circu- 
* Contribution from the Phanerogamic Laboratories of Harvard University, 33- 
Botanical Gazette, vol. 51] : [258 
