The vascular bundle entering the stern from a leaf with a single vein passes by a more or less direct course into the central cylinder of the stem, and does not assume the girdle-like form characteristic of the cycadean leaf-trace.
Mamillaris) there is evidence that they were of the Sigillariopsis type, the leaf being traversed by two parallel vascular strands, derived from the bifurcation of the leaf-trace.
Two leaf-trace bundles started from each angle of the stele, and forked, in passing through the cortex, to supply the veins of the leaf, or its subdivisions.
Each double leaf-trace passes through four internodes before becoming a part of the stele; the double nature of the trace is a characteristic feature.
That these three sterile segments, with their sporangiophores, are together comparable to one of the bracts of Sphenophyllum, with its sporangiophores, is shown by the vascular supply in each case being derived from a single leaf-trace, So far as is at present known, the Sphenophyllales were homosporous.
Araucaria imbricata, &co) the leaf-trace leaves the stele of the stem as a single bundle which splits up into several strands in its course through the cortex.
In the more highly developed lorms, as already indicated, the leaf-trace is split up into a number of strands which leave the base and sides of the leaf-gap independently.
Araucaria, the leaf-traces persist for a considerable time, perhaps indefinitely, and may be seen in tangential sections of the wood of old stems. The leaf-trace in the Coniferales is simple in its course through the stem, differing in this respect from the double leaf-trace of Ginkgo.