The Receptive Surface of the Angiosperm Stigma

Abstract

This paper reports upon a study of the characteristics of the receptive surfaces of angiosperm stigmas, covering almost 1000 species of about 900 genera of some 250 families. In both monocotyledons and dicotyledons the major subdivision is into those stigmas which are 'dry' at maturity, having a hydrated, proteinaceous extracuticular layer or pellicle but no free-flowing secretion, and those which are 'wet' bearing such a fluid secretion when in the receptive state. Further subdivision may be made talcing into account the nature of the surface cells-whether or not the receptive surface is papillate or smooth, and if papillate, whether the papillae are unicellular or multicellular. When the genera are classified according to these criteria, various taxonotnic regularities emerge. Many families are homogeneous in stigma type, but some prove to be diverse. Among these are families such as the Liliaceae, a fact that may have phylogenetic significance. The physiological importance of stigma type is shown by the correlations that exist between the characteristics of the receptive surface and self-incompatibility system. Sporophytic self-incompatibility systems are associated with dry, papillate stigmas; most gametophytic systems with wet stigmas. Further relationships exist with pollen type and physiology. Trinucleate pollen, not readily germinated in vitro, tends to be associated with dry stigmas, while wet-stigma forms tend to have binucleate pollen, easily germinated in liquid or semi-solid media; binucleate pollen, however, occurs with both wet and dry stigmas.