The Structure of the Column in Stylidium

Abstract

In the genus Stylidium, the style and filaments of the flower are fused into a single column. In most species the column, when stimulated mechanically, undergoes a fast firing movement followed by a slow resetting movement. This movement is produced by changes in shape of a normally curved region of the column, the bend. In a wide range of species, the bend has a specialised anatomy and consists essentially of a longitudinal central layer of cells with two distinctive multi-celled layers of thick-walled cells on either side. The thick-walled cells are rich in cytoplasm with amyloplasts and vacuoles of varying sizes, and have non-lignified walls whose cellulose fibrils are arranged approximately transversely. Within the bend, the phloem occurs as discrete small groups of cells separated by some distance from the xylem. In species from the subgenus Centridium both the morphology and the internal structure of the bend differ somewhat from those in most species of Stylidium, and in two species of Stylidium with nonmoving columns, the characteristic cellular anatomy of the bend is entirely absent. The specialised anatomy of the cells and tissues in the bend are clearly associated with the movement of the column. Changes of shape and size of these cells are almost certainly responsible for the change in shape of the bend.