Multimerous patterns

Section 6.4 described multijugate (spiral) systems, while Section 1.2 introduced the idea of whorled patterns. Richards (1951) classified phyllotactic patterns under three headings: spiral, distichous, and whorled. The whorled or verticillate systems have two (the literature proposes the terms decussation, decussate, dimery, dimerous), three (tricussation, tricussate, trimery, trimerous, tristichy, tristichous), or several (tetramery, pentamery, hexamery, …) organs at the same node on the stem, or primordia in the same ring around the center of the shoot apex. This generates vertical rows of organs parallel to the axis of the stem, or rays of primordia in line with the center of the apex, the orthostichies. For uniformity, the suffix “mery” and the prefixes “di, ” “tri, ” “tetra, ”…, and “multi” will be used to express whorls, whether they are superposed or alternating.

The terms distichy or distichous (alternating unimery), and decussation or decussate (alternating dimery) are generally used, and will be used here. In a distichous system, general among monocotyledons, the leaves or primordia are distributed on two opposed orthostichies, one leaf at each node. The decussate system displays a pair of leaves at each node, each pair forming a right angle (when the stem is viewed from above) with the pair on the adjacent node of the stem, thus forming four orthostichies.