The Vascular System of the Wheat Spikelet

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Abstract
The attachment region of a mid-spike spikelet was sectioned serially. These sections were used to construct an accurate 3-dimensional model of the course of the vascular system that supplies the organs of the a and b florets, and the rachilla of the c and d florets. All organs are interconnected by vascular tissue, but certain parts of the system are phloem-only. In particular, the supply to the groove bundle of the pericarp, widely held to be the most important pathway to the grain, is made via an annulus of phloem to which lemma, palea and lodicules have phloem-only connections. The vascular system is sufficiently different from the pattern encountered in vegetative nodes to warrant treatment sui generis. The relationships between different cell types need greater histological study, especially in the complex composite bundles. This analysis shows that bundle shape in cross-section and the arrangement of xylem and phloem vary sharply over very short distances (100 ~ m ) .T he distribution of xylem and phloem transfer cells agrees with the proposal that significant solute relocation takes place in the regions where the vascular supplies to different organs meet. The area in the ovary neck that encompasses the fusion zone of the supplies to lemma, palea and pericarp emerges as a zone in need of detailed study, both in spikelet positions within a cultivar of known, but different, grain performance, and as a region to analyse for inter-cultivar comparisons.