The genus Eugenia (Myrtaceae) in southern Africa: Ontogeny and taxonomic value of the seed

Abstract

Eugenia L. in southern Africa constitutes two coordinate groups of species (groups X and Y). Each group is characterized by a particular type of seed (types X and Y respectively). This study concerns the ontogeny of both seed types. Results are compared with the literature to evaluate its taxonomic significance. In both groups the ovules are hemi-campylotropous, occasionally tending to be ana-campylotropous, sessile or sub-sessile, bitegmic and crassinucellate. Usually only one ovule per ovary matures into an overgrown seed. Abortive ovules are transformed into ovulodes. Ovules in seed type X become amphitropous resulting in sub-reniform or oblong-globose seed. A short funicle is discernible; the hilum is not extended. The testa develops from the outer integument and is relatively thin (0,1 – 0,5 mm), not multiplicative and tends to be exotestal. Two areolae pervaded by vascular strands are present. Secretory cavities develop in the cotyledons. Starch, occasionally with unidentified spherical bodies, is the main storage product. Abundant endosperm is formed. Seed type Y is globose, pachychalazal and sessile with an extended hilum. The testa is relatively thick (0,5 – 1,0 mm), multiplicative, mesotestal and pervaded by vascular strands. Secretory cavities are absent from the cotyledons. Starch is the only storage product. Relatively little endosperm is formed. Generic rank is proposed for both groups. Group X is treated as congeneric with Eugenia s. str. whereas group Y probably represents a new generic segregate. Eugenia L. in suidelike Afrika word deur twee newegeskikte spesiegroepe (groepe X en Y) verteenwoordig. Elke groep word deur ’n besondere saadtipe gekenmerk (tipes X en Y onderskeidelik). Hierdie ondersoek handel oor die ontogenie van beide saadtipes. Die resultate word met literatuur-gegewens vergelyk ten einde die taksonomiese betekenis daarvan te evalueer. Saadknoppe van beide groepe is hemi-kampilotropies (soms geneig om ana-kampilotropies te wees), sittend of half-sittend, bitegmies en krassinusellêr. Gewoonlik ontwikkel slegs een saadknop per vrugbeginsel in ’n uitermate vergrote saad. Die oorblywende saadknoppe aborteer en is blywend. In die geval van saadtipe X word die saadknoppe amfitropies met die gevolg dat die saad halfniervormig of langwerpig-rond is. Die funikulus is baie kort en daar is geen vergroting van die hilum nie. Die relatief dun (0,1 – 0,5 mm) testa ontwikkel uit die buitenste integument, is nie veellagig nie en neig om eksotestaal te wees. Twee areole deurtrek met vaatstringe is teenwoordig. Sekreetholtes ontwikkel in die saadlobbe. Stysel, soms tesame met ongeïdentifiseerde bolvormige strukture, is die hoof-bergingsproduk. Baie endosperm word gevorm. Tipe Y sade is bolrond, pagichalasaal en sittend met ’n vergrote hilum. Die relatief dik (0,5 – 1,0 mm) testa is veellagig, mesotestaal en deurtrek met vaatstringe. Sekreetholtes is afwesig in die saadlobbe. Stysel is die enigste bergingsproduk. Relatief min endosperm word gevorm. Genusrang word vir beide groepe voorgestel. Groep X word beskou as kongeneries met Eugenia s. str., terwyl groep Y waarskynlik ’n nuwe genus verteenwoordig.