Nectaries in Apiales and related groups
Erbar, Claudia; Leins, Peter
Plant Diversity and Evolution Volume 128 No. 1-2 (2010), p. 269 - 295
published: Aug 1, 2010
ArtNo. ESP145012871015, Price: 29.00 €
The stylopodium (the swollen and often strongly expanded base of the style) serves as a nectary in Apiaceae, and this feature (together with leaf and inflorescence characters) has been stressed as a distinguishing character of the family. The position of the nectary is presented for a representative sampling from most of the families and subfamilies currently recognized in the order Apiales.Most Apiales have an inferior ovary with a nectary located on the ovary roof. Inferior ovaries develop from intercalary growth of the floral axis beneath the insertion areas of the perianth members, stamen primordia and dorsal carpel flanks. During this growth, the insertion areas of the carpels become extended and change from a horizontal to a vertical position, while their dorsal flanks remain short at the base and merely contribute to the formation of the ovary roof, style(s) and stigma(s). In the core group of Apiales, comprising Apiaceae, Araliaceae, Myodocarpaceae, Pittosporaceae (suborder Apiineae), only the Pittosporaceae have a superior ovary with a nectary at its base. However, due to intercalary growth in the floral axis, the nectary position in Pittosporaceae corresponds perfectly to that of groups in core Apiales (an example of the principle of variable proportions). Within Apiaceae, two groups exhibit shifts in the position of the nectary. In Apiaceae-Saniculoideae, the nectary tissue can extend to a peripheral position (onto a perigynous hypanthium), and in Actinotus (Apiaceae-Mackinlayoideae), the nectary tissue shifts to the upper part of the style. Among the three small families currently associated with Apiales, Griseliniaceae and Torricelliaceae have a nectary atop the inferior ovary, while Pennantiaceae lack a nectary altogether.Among the euasterids II, nectaries formed by the gynoecium, which mostly is inferior, are a more or less constant feature not only in Apiales but also in Asterales. Most Asterales have an inferior ovary and an ovary-roof nectary. The situation in Menyanthaceae, which have a superior ovary, is comparable to that of Pittosporaceae. In some molecular analyses Apiales form a clade together with Dipsacales, but the two groups differ markedly in the nectary structures. With a few exceptions, the nectaries of the Dipsacales are situated at the inner side of the corolla tube (which may be spurred) or the stamen-corolla tube and consist of one-celled epidermal hairs (in contrast to Apiales and Asterales, where the nectaries are mesophyllary). The only exceptions in Dipsacales are found among three genera in the heterogeneous family Adoxaceae: Adoxa has multicellular trichome nectaries on the petals, Sambucus lacks nectaries, and Viburnum has an inferior ovary with an ovary-roof nectary. Thus within the Dipsacales the nectary conditions typical of many euasterids II is found only in Viburnum. It can be assumed that the ovary-roof nectary in Dipsacales has been lost (probably after the diversification of Viburnum but before that of Sambucus) and that corolla trichomes have taken on the role of nectar secretion. The gynoecial nectary in Viburnum may be considered to be a vestige of that characterizing the early lineages of the asterids, such as Cornales (which have an inferior ovary with nectary atop). It has been preserved in the lineages leading to Apiales and Asterales. But in Aquifoliales (sister to the rest of euasterids II), the wall of the superior ovary contains the nectary.