Reproduction in Seagrasses: Nature of the Pollen and Receptive Surface of the Stigma in the Hydrocharitaceae

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Abstract
This paper describes the characteristics of the pollen and the receptive surfaces of the stigmas in the three marine angiosperms included in the Hydrocharitaceae. The pollen in Enhalus acoroides and Thalassia hemprichii is spherical and has an ornamented exine. An exine layer is not found in Halophila stipulacea where reniform pollen grains are contained within transparent moniliform tubes. Cytochemical tests show that the pollen wall in the three species contains acidic and neutral polysaccharides and acid hydrolase acitivity is detected in the intine of H. stipulacea and T. hemprichii. In Thalassia, one of the intine enzymes, acid phosphatase, is unambiguously associated with cytoplasmic inclusions. Flowering in Thalassia is coincident with the spring tides and the pollen is released as a mass suspended in a thecal slime which contains approximately 5 per cent by weight carbohydrate, the principal mono-saccharide being mannose. Electrophoretic analysis of the pollen-free slime shows a single glycoprotein component. The stigmas of the three seagrasses are papillate and of the ‘dry’ type possessing a continuous protein-aceous pellicle subtended by a cuticle. The stigma pellicle exhibits cytochemically detectable esterase activity and binds the lectin concanavalin A. Acid phosphatase activity is localized beneath the cuticle at the tips of the stigma papillae. The discoveries show that the characteristics of the pollen and stigmas in the seagrasses are comparable with those found in terrestrial flowering plants. The similarity in enzymatic properites of the pollen wall and stigma pellicle suggests that, intriguingly, a similar mechanism of cuticle erosion might well follow compatible pollination both on land and in the sea.