A Theory of the Double Leaf-Trace. Sg 
petiole and rachis, although the middle bundles may approximate 
during some portion of their course—or throughout in certain 
species—so as to simulate a midrib. The midrib found in the 
foliage leaves of many Conifers, is doubtless formed by the more 
or less complete fusion of the central pair, as was seen to be the 
case in Cycads. 1 
Professor Jeffrey, 3 who has drawn attention to the prevalence 
of the double leaf-trace in Gymnosperms, finds that the central 
strand of the Abietineae is nearly always double in origin, at any 
rate in the early leaves. 
The state of affairs seen in simple Cycad leaves (cotyledons, early 
foliage leaves, sporophylls) is precisely that found in the fossil allies 
Poroxylon and Cordaites, in which the double trace dichotomizes 
frequently as it passes up the leaf, and may do so, even before 
entering it. Cordaites, it will be remembered, combines Cycadean 
and Coniferous characters, so that it is interesting to find in it a 
form of nervation which is the basis of that of the modern Cycads 
and Conifers, and which is retained in its simplicity by some of the 
Araucarieae. 
The double leaf-trace of the extinct group of the Cycado- 
Filices is one of its most marked characteristics. 
Dr. Scott in his “ Studies in Fossil Botany” (1900) commented 
on the frequent occurrence of a double leaf-trace in the Cycado- 
Filices and the older Gymnosperms, and all the evidence brought 
to light since then goes to show that this character is more funda¬ 
mental and widespread than was then supposed. The Gymnosperms 
and Cycado-Filices have been brought so close together during the 
last few years by the discovery of the seeds of the latter group, 
that we cannot now doubt that the paired parallel strands of the 
simpler Gymnosperms are the lineal descendants of the double 
leaf-trace of the Cycado-Filices. It is of considerable interest 
therefore to find the last remnant of the double leaf-trace in the 
cotyledons of Angiosperms. It furnishes another link to bind this 
puzzling modern group to the long and ancient series arising from a 
fern-like base. 
It is not likely that any Ferns now known, either recent or fossil, 
give us much guide to the actual form from which the Cycado-Filices 
i In Cycas Seemanni one cotyledon may have two central 
bundles, while the other has a single central bundle 
(Bower, 1884). 
E. C. Jeffrey. Comparative Anatomy and Phylogeny of the 
Coniferales. Part 2, Abietineae. Memoirs of the Boston 
Soc. of Nat. His., 1905. 
