Upon this torus the parts of the flower are arranged in a crowded manner, usually forming a series of verticils, the parts of which alternate; but they are sometimes arranged spirally especially if the floral axis be elongated.
This suppression when confined to the parts of certain verticils makes the flower asymmetrical.
Thus, in many Caryophyllaceae, as Polycarpon and Holosteum, while the calyx and corolla are pentamerous, there are only three or four stamens and three carpels; in Impatiens Noli-me-tangere the calyx is composed of three parts, while the other verticils have five; in labiate flowers there are five parts of the calyx and corolla, and only four stamens; and in Tropaeolum pentaphyllum there are five sepals, two petals, eight stamens and three carpels.
The parts of the several verticils often differ in their mode of aestivation.
Using Calamites as a generic name for all those Calamarian stems in which the ribs alternate at the nodes, we have, on Weiss's system, the following sub-genera: Stylocalamites, branches rare and irregularly arranged; Calamitina, branches in regular verticils, limited to certain nodes, which surmount specially short internodes; Eucalamites, branches present on every node.