Pollen-Stigma Interaction in the Leguminosae: the Organization of the Stigma in Trifolium pratense L.

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Abstract
The intact stigma of Trifolium pratense possesses a smooth receptive surface fringed by a few ranks of brush hairs. This surface is ensheathed by a thin (75–100 nm) but highly impermeable cuticle, which encloses four to five ranks of secretory cells immersed in their secretory products. Experimental single-grain pollinations show that pollen cannot become hydrated or germinate on the intact surface. The cuticle is ruptured when the flower is tripped; the secretion is released, and captured pollen-self or cross-can then germinate. As in other papilionoid Leguminosae, this mechanism provides a guard against premature selling. The secretory cells are elongated; they remain in communication through persistent pit-fields as the intercellular spaces fill with secretion product. The secretion forms a lipid-rich emulsion, with a mucilaginous aqueous phase which reacts cytochemically for protein and carbohydrate and has esterase activity. During the early development of the stigma head, the cells possess a fine-structure appropriate to their secretory function, with abundant ribosomal and smooth endoplasmic reticulum, stratified or in the form of ramifying and anastomosing tubules, numerous mitochondria and a well developed Golgi system. Lipid globuli, partly invested in endoplasmic reticulum, are abundant in the young cells, but there is as yet no indication of how the lipid is transferred to the intercellular spaces during the secretory period. As the stigma matures, the secretory cells become moribund.