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

In seeds of Trifolium repens, T. pratense, and Lupinus arboreus,the hilum is a hygroscopically activated valve in the impermeableepidermis of the testa. When relative humidity was low the fissurein the hilum opened permitting the seed to dry out; when therelative humidity was high the fissure closed obstructing theabsorption of moisture. During seed-ripening the moisture contentfell readily to approximately 25 per cent., and thereafter moreslowly until the epidermis became impermeable at approximately14 per cent, moisture content. Further drying of the seed tookplace only by diffusion of water vapour through the hilum. ‘Hard’seeds tended to have a moisture content in equilibrium withthe lowest relative humidity to which they had been exposed.They absorbed moisture under conditions of gradually increasingrelative humidity such that the hilar fissure remained open.The duration of the impermeable condition increased with thedegree of desiccation brought about by loss of water throughthe hilum.