The Vascular System in the Rachis of a Wheat Ear

The vascular system in the rachis of ears of wheat (Triticumaestivum L. cvs Gamenya, Olympic and Bungulla) was examinedon material grown in the field and in a growth cabinet. In theinternodes, central and peripheral bundles were observed andtheir mean number and size were determined. A significant 1:1 relationship between the number of spikelets on the ear andthe number of central bundles at the base of the rachis wasestablished. The number of both central and peripheral bundlesdeclined acropetally along thé length of the rachis.The decline in peripheral bundles occurred mainly between internodes1 and 6, numbered from the base. The decline in central bundlesoccurred at a rate of less than one bundle per internode betweeninternodes 1 and 4, though in some ears, there was no decline;in larger ears, central bundles declined at a rate of one totwo bundles per internode between internodes 5 and 11. Aboveinternode 11, the rate of decline varied with ear size. Threecentral bundles consistently reached the terminal spikelet.The number and cross-sectional surface area of xylem vesselsand sieve tubes and the total vascular size also declined acropetallyalong the rachis. The decline in total vascular size was dueto (a) some bundles branching and to reductions in size, (b)the diversion or dropping of bundles into spikelets, or (c)a combination of (a) and (b). These observations are discussedin relation to the distribution of grain number and weight onthe ear. Triticum aestivum L., wheat, rachis, spikelets, vascular anatomy, xylem, sieve tubes