The rachilla in the cob is commonly being called the pedicel while actually the pedicel is deeper in the cob and erroneously being called the spikelet trace. The difference is important because the onset of spikelet development is a regulatory switch point characteristic of the grass genotype. Glossaries define the pedicel as the foot stalk which elevates the pedicellate spikelet above the sessile one. The rachilla is the floral axis within the spikelet. In maize, the rachilla usually carries a pair of florets, each with a lemma and a palea. The lower floret on the ear does not usually develop. The relative lengths of the pedicel and the rachilla are inherited traits that are under separate genetic control.
On a larger scale of elaboration, the homologues of the pedicel and the rachilla are the uppermost internode of the shank and rachis or cob which terminates it. Like the spikelets, the ears may be paired with one sessile and the other pedicellate.
The correct usage of the terms pedicel and rachilla for the male spikelets in maize is clear as it is in most grasses. But when the terms are applied to the ear, there is confusion perhaps because of the extreme condensation, fusion and distortion of parts. The jargon "tip cap" of the kernel refers to a broken fragment of the rachilla together with its "chaff." The pedicel (spikelet trace) is fused into the floor of the cupule and sometimes buried further by a fusion of the roof of the cupule down onto the floor. The length of the rachilla together with the length of the pedicel and its associated cupule as well as the pith area constitute the thickness of the cob.
Walton C. Galinat
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