The Function of the Hilum in Some Papilionaceae in Relation to the Ripening of the Seed and the Permeability of the Testa

Abstract

In seeds of Trifolium repens, T. pratense, and Lupinus arboreus, the hilum is a hygroscopically activated valve in the impermeable epidermis of the testa. When relative humidity was low the fissure in the hilum opened permitting the seed to dry out; when the relative humidity was high the fissure closed obstructing the absorption of moisture. During seed-ripening the moisture content fell readily to approximately 25 per cent., and thereafter more slowly until the epidermis became impermeable at approximately 14 per cent, moisture content. Further drying of the seed took place only by diffusion of water vapour through the hilum. ‘Hard’ seeds tended to have a moisture content in equilibrium with the lowest relative humidity to which they had been exposed. They absorbed moisture under conditions of gradually increasing relative humidity such that the hilar fissure remained open. The duration of the impermeable condition increased with the degree of desiccation brought about by loss of water through the hilum.