On the Relationship of the Apocynaceae and Periplocaceae

Abstract

It is generally agreed that the Periplocaceae arose from apocynaceous ancestors. Whether these ancestors were from the Plumerioideae, Cerberoideae or Apocynoideae has long been in dispute. This investigation is a comparative study of the pollen morphology and floral structure of selected genera of Apocynaceae and Periplocaceae. The examinded genera of the Plumerioideae and the Cerberoideae have 3-colporate, smooth to sculptured pollen grains. In the Apocynoideae there are single, smooth, 2–3-porate pollen grains or 3-to multiporate grains loosely united in tetrads of variable size and shape. The exine consists of an outer thick, homogeneous stratum subtended by a very thin, granular-fibrillar stratum, and globules; the internal septa are double-layered due to partial fusion of the tecta of the individual grains. The genera of Periplocaceae have pollen grains united in elongated, rhomboidal to rounded, or tetrahedral and decussate tetrads with few to many pores, pairwise, opposite or irregularly arranged. The exine consists of a homogeneous outer stratum and a thin granular stratum below. The internal walls (septa) are relatively thin, and perforate. Hemidesmus has an aggregation (massula) of elongated tetrads with indistinct pores. The exine is divided into a compact outer stratum and an inner, fairly thick, granular stratum. The inner walls (septa) are thin, granular, and non-perforated. Trends in floral structure center around increasing precision of the pollination mechanism for transport of pollen en masse. In the Apocynaceae this is mainly effected by the androceum and gynoecium. In the Periplocaceae the corona is also incorporated. Porate tetrads are correlated to a large extent with the advent of translators. Style-head secretions evolve from foamy adhesive for the transport of single pollen grains in the Apocynaceae to firm translators transporting pollen tetrads in the Periplocaceae and in Forsteronia, Apocynum, Poacynum and Trachomitum. The latter three genera are so similar in all respects that recognition of more than one genus seems unjustified. With special reference to pollen morphology a possible trend of evolution is postulated from taxa with single, 3-colporate grains to 3-porate grains and from taxa with pollen united in tetrads with compact inner walls as in the Apocynoideae, to taxa of the Periplocaceae with tetrads perforated septa and to taxa with pollen (tetrads) in masses. The Periplocaceae appear unrelated to the Plumerioideae and the Cerberoideae. The similarity between tetrads of Apocynum s.l. and Periplocaceae is here regarded as parallel evolution within two closely related lineages.