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

The interconnecting system of leaf traces constitutes only afraction of the vascular tissue found in the base of a ryegrassstem. The tillers and nodal roots have their insertion in thiscongested region of the plant in addition to the majority ofthe leaf traces. Study of the attachment of nodal roots revealsthe extensive differentiation of vascular tissue in a perforatedcylinder surrounding the inner leaf trace system—the peripheralplexus. This plexus consists of two components, the diffuse bundlesorientated along the stem axis, and interconnected with theroot girdles orientated around the stem axis. The peripheralplexus which is bounded externally by a mestome sheath, makesnumerous contacts with the leaf trace system within it, bothdirectly and via the nodal plexi, and receives the vascularattachment of all the nodal roots. It appears in the stem atabout the same time as adjacent nodal roots, and differentiatesfrom meristematic tissue totally independently of the leaf tracesystem. The diffuse bundles themselves apparently differentiateacropetally in this meristematic tissue and are augmented bybranches from leaf traces and the nodal plexi. The integrated vascular systems of leaf, stem, and root at thebase of the grass plant, bounded by a mestome sheath, must allowtotal intercommunication between all organs. Nearly all tissuewithin the mestome sheath is vascular in nature and it is intothis vascular tissue that the leaf traces associated with transfercells are inserted.