A Theory of the Double Leaf-Trace. 83 
Investigation of seedling structure in other Gymnosperm groups 
also proves of great interest. 
The Araucarias approach the Cycads very nearly in some of 
their features, and the cotyledons have each four bundles, which 
towards the base approximate in pairs composed of one central and 
one lateral. They thus enter the hypocotyl from the diagonal corners. 
This grouping is also found in Dicotyledons ( Fergus , Quercus, 
Achras), and almost certainly in some Cycads. Leaving out of 
account for the moment the variations met with in the very swollen 
forms, the hypocotyl bundles then form a diarch root by the 
approximation of the protoxylem groups in the cotyledonary plane. 
The seedlings of the Coniferae proper are in many cases com¬ 
plicated by the phenomenon of polycotyledony, of which a very 
interesting explanation—which the present work fullly bears out— 
was given by Mr. Hill at the York meeting of the British 
Association. Allowing for the complications thus induced, the 
polycotyledonous forms conform to the Coniferous type about to 
be described. 
In the groups having two cotyledons, viz., the Taxaceae, 
Cupressineae, and some of the Taxoideae, the cotyledons each 
possess a single somewhat extended bundle, which forms a pole of 
the diarch root, precisely in the same way as does the double 
bundle of some Dicotyledons and the two separate bundles of other 
Dicotyledons and of Cycads. 
III.— The Case for Reduction in Gyainosperms. 
That the Coniferous forms show reduction on the Cycad type 
is a reasonable hypothesis on the grounds of the generally acknow¬ 
ledged antiquity and primitiveness of the latter group, but there is 
additional evidence which raises it to a reasonable certainty. 
Ginkgo biloba, which has proved so interesting and important 
in other directions, is no less so in its seedling structure. Each 
cotyledon is provided at the base with a double bundle, the halves 
of which separate widely as they pass up the petiole, and in the 
lamina each proves to be itself double, i.e., composed of a central 
and a lateral bundle. Thus we have the representatives of the four 
bundles characteristic of the Cycads, and grouped during one 
portion of their course as in the Araucarias and some Dicotyledons. 
The hypocotyl is characterized by a very high transition, giving 
rise to a diarch root, but it passes through a fugitive tetrarch stage. 
The members of the Taxaceae form a diarch root from an 
