The opening mechanism of follicles of some species of Banksia

Abstract

Banskia ornata F. Muell. is a common dominant in heaths in south-eastem Australia that are frequently swept by fires which kill the plants. Regeneration is only from seed, suggesting that the future of the species is almost completely fire-dependent. At maturity, a multiple fruit is formed consisting of a number of follicles attached to a central rachis. Each valve of the follicle consists of an exocarp comprising an epidermis with hair cells and some sclereid bundles; a mesocarp containing bundles of sclereids in which the microfibrils are almost transverse to the major cell axis; and an endocarp in which the innermost cells are fibrous in form and in which the microfibrils of cellulose are oriented mainly parallel to the longitudinal axis of the cell.

Also present in the endocarp but abaxial to the fibrous sclereids are sclereid bundles that show similar microfibril orientation to the mesocarp sclereids. The junction between the valves of the follicle has a layer of interdigitating cells. A resinous substance together with phenols are present in and between these cells. Because of the difference in structure of the sclereids of the endocarp and of those of the mesocarp and exocarp, stresses develop between these tissues on desiccation. Events leading to the opening of the follicles are: (1) at maturity resin and phenols are secreted into the vascular elements at the base of the rachis, severing the vascular supply to the follicle; (2) with the advent of fire, the resin at the junction of the valves of the pericarp is destroyed, releasing the stress in the valves of the follicles, which reflex, allowing the release of seed. In B. marginata and B. Integrifolia, the anatomy of the valves of the follicle is basically similar to that of B. ornata. However, the follicles open without the intervention of fire. Thin-layer chromatography has shown that the resins extracted from the cells of the junction zone between the valves of the follicle of B. ornata are chemically different from those extracted from the non-fire-dependent species, suggesting that they may differ also in their physical properties.