The Vascular Pattern of Italian Ryegrass (Lolium multiflorum Lam.): 3. The Leaf Trace System, and Tiller Insertion, in the Adult

The leaf trace system in the region of congested internodesat the base of Lolium multiflorum is described. A typical major trace in a leaf consists of a collateral bundlehaving a double bundle sheath and incorporating a certain amountof sclerenchyma. As such a leaf trace is followed down intothe stem it increases in diameter, loses the inner (mestome)bundle sheath, and the xylem becomes associated with xylem transfercells. Lower down, the bundle diameter is reduced although nowit has become amphivasal. The internal xylem only is still associatedwith transfer cells. The proximal portions of the bundle aremuch reduced, transfer cells, mestome sheath, and sclerenchymaare lacking and the now insignificant bundle merges with a lowerleaf trace or some other vascular tissue. Such a bundle in thestem may be in direct contact via bridges with other leaf traces,with the nodal plexus, and with the peripheral plexus that surroundsthe inner leaf trace system. In the base of a typical young plant, approximately one-halfof all leaf traces, including all the median veins, join bundlesfrom the next oldest leaf. Approximately one-third join thenodal plexus, and the remainder variously join bundles fromthe same or next but one oldest leaf to join the peripheralplexus. The differentiation of tiller insertions into the pre-existingmain stem system is highly variable. In a very young tillera number of traces were seen to terminate before the main systemwas reached suggesting basipetal differentiation. The actualconnections made by the tiller traces may occur with any nearbyleaf trace, the nodal plexus, or with the peripheral plexus.Later differentiating leaf traces in a tiller join leaf tracesof the tiller itself. Occasional bundles from secondary tillers by-pass the vasculartissue of the primary tiller to join directly with that of theparent plant. Vascular connections between parent and tiller,although very variable, appear to be totally comprehensive froma functional standpoint.