Anatomy and Movement of The Column in Stylidium

Abstract

The flower of the trigger-plant Stylidium has stamens and style fused into a single column. A bend in the column, upon suitable stimulation by nectar-gathering insects, reverses its curvature in 10-30 ms, depending on species, causing the column to flip rapidly through an angle of 2-5 radians from its normal position against the anterior modified petal, the labellum, until it comes to rest against the posterior petals. This rapid movement or 'firing' provides a mechanism for cross-fertilization of the flowers. The column, after this rapid excursion then slowly resets to its original position in times from 200 to 700 s. Detailed measurements of the time courses of firing and resetting in several species of Stylidium have been made. During the firing, except for a short period of 1-10 ms in which it accelerates from rest under a force up to 50 g, the column moves with approximately constant angular velocity. This characteristic linear behaviour contrasts markedly with the distinctly non-linear time course of resetting. The dependence of firing and resetting rates on temperature has been investigated. A light-microscope study of the anatomy of the column shows that the mobile bend region possesses an internal anatomy strikingly different from the rest of the column. The bend consists of an inner core of three distinct longitudinal layers of thick-walled collenchymatous cells, surrounded by parenchymatous cells. A model in which changes in length pf the anatomically posterior outer layer of the core are greater than in the other layers is proposed to account for the movement of the column.