Vascular System of the Fertile Floret of Cynodon dactylon (Gramineae- Eragrostoideae)

Abstract

A facsimile of the vascular system of the fertile floret of Cynodon dactylon (Eragrostoideae) was reconstructed from transverse sections of the spikelet. Three collateral bundles, A, B, and C, cluster in the rachilla at the floret base. Bundles A and B are, respectively, basipetal continuations of each trace from the left and right lodicules. Bundle C is a basipetal continuation of the extension of the rachilla into an aborted floret behind the palea. The lemma and palea traces basipetally combine variously with these three rachilla bundles. Relative to genera outside the Eragrostoideae, the bundles in the lower rachilla of C. dactylon remain discrete. However, the vascular component in the rachilla related to the three stamen insertions is a unified one, the upper plexus, consisting of a core of abnormal tracheary elements (the xylem discontinuity) surrounded by a network of sieve elements. This upper plexus is connected, proximally, to the bundles in the lower rachilla at the entrance of the lodicule traces. Distally, an amphicribral bundle from the vascular component related to the pistil, the pistil plexus, merges into the top of the upper plexus among the stamen insertions. The pistil plexus is a horizontal disc of sieve elements surrounding a xylem discontinuity at the pistil base. To the anterior of the pistil plexus attach the collateral, stylar bundles. From the posterior of the pistil plexus emerges the placental bundle, which, distally, merges with the ovule in the placenta. The presence of the xylem discontinuity suggests an affinity between the Eragrostoideae and the Festucoideae However, other aspects of the floret vascular system of C. dactylon, like the absence of a plexus from the lower rachilla, indicate the distinctiveness of the Eragrostoideae.