The Hilar Region in Leucaena leucocephala Lam. (De Wit) Seed: Structure, Histochemistry and the Role of the Lens in Germination

Abstract

Structural and histochemical features of the hilar region in the seed of Leucaena leucocephala Lam. (De Wit) were investigated by bright-field and fluorescence light microscopy and scanning electron microscopy. In the dry seeds the lens appeared as an elliptical depression prevailing in size over the hilum and micropyle. In the imbibed seeds, both naturally softened seed and those requiring softening with boiling water, the lens rose gradually to form a cavity between the palisade layer and the mesophyll below. This apoplastic pathway was traced using Ruthenium Red dye. In place of the hour-glass cells, the hilar region contained abnormal cells, called 'white cells' because they remained unstained by Toluidine Blue O at pH 4·4. The apparently opposing characteristics of the lens are discussed. It is hypothesized that the lens acts as a valve. By remaining closed it hinders the entry of water because of the presence of a high light-line rich in callose and in lipid-like substances and to the almost total lack of hydrophilic polysaccharide material. Under favourable conditions for germination, the lens opens, allowing water to enter, through a thin palisade layer and with the probable intervention of the 'white cells'.