Development of the vascular system in the upper rachilla of the fertile floret of Anthoxanthum has been described. The sieve-element plexus, the xylem discontinuity, and the stamen traces constituted this system. The plexus developed in two parts: (1) an upper, circular continuum whose procambium was continuous with the procambium of rachilla bundle 3; and (2) a lower, posterior semicircle, which developed later but acropetally and continuously from rachilla bundles 1 and 2. Sieve elements also developed acropetally and continuously from bundle 3 to supply the upper, circular continuum and subsequently from bundles 1 and 2 to supply the lower, posterior semicircle. Procambium for each stamen trace was also continuous with bundle 3. Sieve elements for the stamen traces developed acropetally and continuously from the upper, circular continuum. Anomalous tracheary elements for the xylem discontinuity were first initiated in three separate sites: one developed acropetally and continuously just above normal tracheary elements of bundle 3, and one near each of the insertions of the two stamens. Later the anomalous tracheary elements of these sites became initially continuous in a Y-shaped xylem discontinuity. Subsequent development of the anomalous tracheary elements of the flanking rachilla strands, which linked the plexus and gynoecium, added a left and right branch to the xylem discontinuity. In more mature stages, obliteration of older, anomalous tracheary elements, plus differentiation of intergrading anomalous tracheary elements and thick-walled parenchyma, contributed to the heterogeneity of the xylem discontinuity.
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Robinson‐Beers et al. (1987) studied this question.