A leaf-trace, as it passes through the cortex, has a collateral structure, the protoxylem being situated at the inner edge of the xylem; when it reaches the leaf-base the position of the spiral tracheids is gradually altered, and the endarch arrangement (protoxylem internal) gives place to a mesarch structure (protoxylem more or less central and not on the edge of the xylem strand).
Similarly in the sporophylls of some cycads the bundles are endarch near the base and mesarch near the distal end of the stamen or carpel.
It consists of a ring of endarch collateral bundles1 surrounding a hollow pith.
The constitution of the stele of a flowering plant entirely from endarch collateral bundles, which are either themselves leaf-traces or will form leaf-traces after junction with other similar bundles, is the great characteristic of the stem-stele of flowering plants.
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