The pollen-stigma interaction in the grasses. I. Fine-structure and cytochemistry of the stigmas of Hordeum and Secale

The papillar cells of the stigmas of Secale cereale and Hordeum bulbosum show characteristics suggesting specialisation for both external and internal secretion. Each papilla terminates in a reflexed pollen-receptive tip bearing a proteinaceous surface pellicle overlying a thin mucilaginous layer, which in turn adjoins the markedly discontinuous cuticle. The basal part of each papilla contributes to the axis of a secondary stigma branch. A distinct layer of the papilla wall underlaying the cuticle is continuous with interstitial material in the axis of the branch. This material, which forms the pollen-tube transmitting medium, is constituted principally of acidic pectic polysaccharides, but contains a protein component, seemingly an internal secretory product of the basal parts of the branch cells. The papillar cells are bounded by a further distinctive wall layer. This is PAS reactive, stains with phosphotungstic acid-HCl, and also with the fluorescent polysaccharide stain, calcofluor white, which has been considered to have affinity with 1,4-linked glucans. The layer shows no evidence of a microfibrillar structure, however, and is not birefringent. Dictyosomes are present during the final maturation of the stigma papillae, but they show no special evidence of secretory activity. On the other hand, paramural bodies are frequent in the papillar cells, and in the branch axes they are usually associated with vesicle swarms in the adjacent intercellular spaces. It seems likely that the paramural bodies form a granulocrine secretory system producing both the surface secretions of the papillae and the protein-polysaccharide constituents of the intercellular spaces in the branch axes. The paper is the first of two; the second will be concerned with the responses of the stigma to pollination.