THE MODIFIED STOMATA OF THE FLORAL NECTARY OF VICIA-FABA L .1. DEVELOPMENT, ANATOMY AND ULTRASTRUCTURE

This study of the floral nectary of Vicia faba L. focusses on the modified stomata through which nectar flows, and on the question of whether they can regulate their stomatal pore aperture. The floral nectary of V. faba consists of a disk which surrounds the gynoecium. As the flower bud matures, a tapered projection arises by cell divisions and expansion at a position opposite the free stamen. Modified stomata develop from pentagonal guard mother cells on this projection, and are often contiguous, but because development of modified stomata is asynchronous, many stages can occur in a local area. Plasmodesmata, which connect guard mother cells and guard cells of developing modified stomata with adjacent cell types, become covered by cell wall material on the guard-cell side in mature modified stomata. The numerous small vacuoles of guard mother cells and young guard cells are replaced by a single large one as the guard cells expand. Pore development occurs concurrent with this expansion as the ventral walls separate at a designated location leading to rupture of the inner, and then outer, cuticles, to reveal the pores of mature modified stomata. The stomata are considered to be "modified" because the guard cells of mature stomata only rarely can contract sufficiently to close their pores. Several structural features may contribute to this inability to close the pores by guard-cell movements: a cuticle which lines all free surfaces of guard cells; ridges of circumferentially-arranged microfibrils along the outer (and sometimes inner) walls; an unidentified osmiophilic wall material. Pores of modified stomata reach their maximum mean aperture dimensions a few days before the onset of secretion. Also, open, and even immature, modified stomata occur on old post-secretory glands undergoing degradation. Together with their inability to regulate, the lack of close correlation between development of modified stomata and secretion suggest that nectar flow is not finely regulated by these structures. Although there is no evidence for regulation of pore aperture, pores may become occluded. In these cases, an osmiophilic material, released through channels in the cuticle of outer and ventral walls, is deposited across the pore as a film. Although occluded pores can occur on very young buds, they are more common on secretory or post-secretory nectaries.