Pollen morphology with reference to the taxonomy and phylogeny of the Monochlamydeae

Abstract

The Monochlamydeae are an artificial group of dicotyledons possessing apetalous flowers and are heterogeneous pollen morphologically. The families Aristolochiaceae, Piperaceae, Chloranthaceae and Myristicaceae with monocolpate pollen grains, Casuarinaceae, Urticaceae (including Moraceae and Cannabinaceae), Myricaceae and Betulaceae, with triporate grains, Salicaceae and Fagaceae with tricolpate grains, Amaranthaceae, Juglandaceae, and Chenopodiaceae with panporate grains are stenopalynous. Among the eurypalynous families, the highest development of eurypalyny is found in the Euphorbiaceae. Among the dicotyledons, monocolpate pollen grains occur in the ranalian families Magnoliaceae, Nymphaeaceae and Annonaceae (partly) and monocolpate grains in some of the Monochlamydeae present an evidence of its relationship with the ranalian families. In considerations of pollen-spore evolution, the present author is of the opinion that the triletous (trichotomocolpate) form is the basic type, from which evolved all other apertural types possibly along two main lines namely the tricolpate stock and the monocolpate stock. Among the ranalian families, the monocolpate stock is represented by the Magnoliaceae, Annonaceae and the Nymphaeaceae, and the tricolpate stock is represented by the Ranunculaceae (partly). In such a scheme, the monocolpate stenopalynous families of the Monochlamydeae (viz. Piperaceae, Chloranthaceae, Myristicaceae and Aristolochiaceae) may be considered to be phylogenetically related to the magnolian stock of the Ranales. The other families of the Monochlamydeae (either stenopalynous or eurypalynous) seem to have evolved along parallel lines, but are possibly connected to the ranunculacean stock having the basic tricolpate pollen forms. The apertural evolution in the tricolpate stock is considered to be in the order ( 1) zonicolpate → zoniporate ( 2) zonicolpate → pancolpate → panporate ( 3) trizonicolpate (by reduction) → inaperturate; or from other forms → inaperturate. In such a scheme, the tricolpate sporomorphs within every family of the Monochlamydeae are to be considered more primitive. Other forms have evolved along the lines mentioned above.