Morphology, ultrastructure, and function of extrafloral nectaries in three species of Caesalpiniacae

Light and electron microscopy reveal that the morphologically well-differentiated petiolar nectaries of Ckamaecrista fasciculata, Senna hepecarpa, and S. marilandica have an unusually simple anatomy consisting of an epidermis immediately subtended by a mass of small, loosely-packed parenchyma cells. Vascular strands from the petiolar bundles enter the nectary and terminate as phloem within or near this parenchyma. In mature, secreting nectaries, the cuticle of the epidermis extends between the epidermal cells and into the nectary parenchyma, where it occupies, but does not occlude, much of the free space of this tissue. The cutin is not found below the level of the phloem endings and it is not found in very young nectaries, but begins to appear when cell expansion occurs. These observations, together with the proximity of phloem to the parenchyma free space and the almost exclusive presence of sucrose in the nectar suggest that, although symplastic transport of sugars may occur, an alternate pathway for secretion is possible whereby sugar diffuses from the phloem, moves through the nectary to the surface without being acted upon by cells in transit, and is released by rupture of the external cuticle and the concomitant activity of foraging ants and other nectar feeders.