Senecio squalidus (Asteraceae) has been shown to possess a stigma with characteristics of both “dry” and “wet” types of stigma. The “semidry” stigma of Senecio is characterized by the presence of a surface cuticle overlaid by a proteinaceous pellicle and a small constitutive surface secretion consisting of lipid, carbohydrate, and protein. We anticipate that this semidry stigma may be a general feature of the Asteraceae. Secretion by the Senecio stigma is enhanced after both compatible and incompatible pollinations, when material secreted by the stigma combines with pollenkitt extruded from the alveolar exine of the pollen to form a heterogeneous “attachment foot” between pollen and stigmatic papillae. During this period, discrete inclusions, “wall bodies,” can be seen within cell walls of papillae in contact with pollen grains, apparently exporting their contents across the cell wall and onto the surface of the stigma. Following compatible pollination, the emergent pollen tube grows through the attachment foot and between the tightly packed stigmatic papillae before penetrating the stigma at the base of the papilla cells, where the cuticle is absent. The pollen tube then grows intercellularly, within the middle lamella, through the stigma toward the style. Following incompatible pollinations, development of pollen is highly variable. Most incompatible pollen grains fail to germinate, but many do germinate to produce pollen tubes, some of which penetrate the stigma before they are inhibited. Such extensive development of incompatible pollen tubes is unusual for a species with homomorphic sporophytic self-incompatibility. These observations are discussed as a comparison with events at the dry stigma surface of Brassica following compatible and incompatible pollinations and in relation to current theories on the evolution of wet and dry stigmas.
