The Vascular Pattern of Italian Ryegrass (Lolium multiflorum Lam.) 4. The Peripheral Plexus, and Nodal Root Insertion

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Abstract
The interconnecting system of leaf traces constitutes only a fraction of the vascular tissue found in the base of a ryegrass stem. The tillers and nodal roots have their insertion in this congested region of the plant in addition to the majority of the leaf traces. Study of the attachment of nodal roots reveals the extensive differentiation of vascular tissue in a perforated cylinder surrounding the inner leaf trace system—the peripheral plexus. This plexus consists of two components, the diffuse bundles orientated along the stem axis, and interconnected with the root girdles orientated around the stem axis. The peripheral plexus which is bounded externally by a mestome sheath, makes numerous contacts with the leaf trace system within it, both directly and via the nodal plexi, and receives the vascular attachment of all the nodal roots. It appears in the stem at about the same time as adjacent nodal roots, and differentiates from meristematic tissue totally independently of the leaf trace system. The diffuse bundles themselves apparently differentiate acropetally in this meristematic tissue and are augmented by branches from leaf traces and the nodal plexi. The integrated vascular systems of leaf, stem, and root at the base of the grass plant, bounded by a mestome sheath, must allow total intercommunication between all organs. Nearly all tissue within the mestome sheath is vascular in nature and it is into this vascular tissue that the leaf traces associated with transfer cells are inserted.

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