n. In botany: In an anatropous or amphitropous (hemitropous) ovule or seed, the adnate cord which connects the hilum with the chalaza, commonly appearing as a more or less salient ridge, sometimes completely embedded in a fleshy testa of the seed. See cuts under anatropous and hemitropous.n. A longitudinal line or rib on the valves of many diatoms, connecting the three nodules when present. (See nodule.) The usual primary classification of genera depends upon its presence or absence.n. In anatomy, a seam-like union of two lateral halves, usually in the mesial plane, and constituting either a median septum of connective tissue or a longitudinal ridge or furrow; specifically, in the brain, the median lamina of decussating fibers which extends in the tegmental region from the oblongata up to the third ventricle.n. In ornithology, the groove along the under side of the rachis of a feather.