| Summary: | The gynoecia of all three species considered in this study originate not as individual carpel primordia but rather as a continuous ring. Processes of continued radial expansion and differential zonal growth give rise to the mature body of the gynoecium with no observable fusion taking place. Later manifestations of distinctness and separation of carpels represent secondarily acquired traits. Consequently, the mature gynoecia of the Malvaceae cannot be reliably used to infer early developmental events.
The physical environment in which carpels originate is proposed to play a role in determining carpel number. Carpel size at inception does not vary considerably among the different species surveyed here. Ring size, however, does and this presumably dictates carpel number by the upward limit of what its circumference can ultimately accommodate.
The uniovulate condition appears to be derived from the pluriovulate one through several interrelated developmental events. On the basis of acropetal initiation of ovules in Abutilon species and the precocious development of style primordia in Malacothamnus fasciculatus, a mechanism for the origin of the uniovulate carpel is proposed. Here, early style growth may limit zonal growth of the gynoecial base so that the acropetal series of ovule initiations is disrupted, leaving only a single basal one.
The study of gynoecial development in this group has been hindered by certain problems of interpretation (e.g., Duchartre, 1845; Klotz, 1975; present account). These include difficulties in conceptualization of developmental processes and their reconciliation with preconceived views of the evolutionary origin of gynoecia. Consideration of relative size among successive stages is crucial, since the affect of radial growth is otherwise easily overlooked.
Despite the differences of their mature gynoecia, the three species studied were determined to be strikingly similar in development, thereby supporting the close affinities attributed to them. In the final analysis, however, conclusive statements regarding the systematic implications of the ontogenetic patterns observed would be premature. Too few taxa have been studied and those that have should be reassessed in light of the developmental phenomena presented here. === M.S.
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