Sextonia, a New Genus of Lauraceae from South America

Abstract

The genus Sextonia is described; it consists of two species, the rather widespread S. rubra and the new species S. pubescens, restricted to eastern Peru. Ocotea is the largest genus of neotropical Lauraceae, including at least 300 species (van der Werff, 1991). It is not surprising that with such a large number of species the genus shows considerable morphological variation, is difficult to delimit, and is in much need of a modern revision. Oftentimes the genus has been used to accommodate species that could not be adequately placed in other, better defined genera. As information from other disciplines, most notably wood and bark anatomy (Richter, 1981), has become available, some of the anomalous species have been removed from Ocotea and recognized as distinct genera. Rohwer et al. (1991) created the genus Chlorocardium for two species previously placed in Ocotea and discussed some other problematical species. I propose to recognize one of those species, until now treated as Ocotea rubra, as belonging to a distinct genus and will describe a new species of this genus as well. Sextonia van der Werff, gen. nov. TYPE: Sextonia rubra (Mez) van der Werff. Figure 1. Arbores magnae. Folia ad apices ramulorum aggregata. Inflorescentiae axillares, racemosae vel paniculatae. Tepala 6, inaequalia, 3 exteriora quam 3 interiora minora, erecta; stamina 9, 4-locellata, 6 exteriora locellis in arcu non profundo dispositis, 3 interiora locellis in 2 paribus superpositis; staminodia 3; receptaculum profundum. Fructus cupula insidens, ab ea 13 vel fere omnino obtecta. Large trees, to 45 m tall. Twigs thick, corky, terete. Leaves alternate, clustered near the tips of the branches, pinnately veined, without domatia. Inflorescences axillary, racemose or paniculate, ultimate divisions of inflorescences frequently not strictly cymose, but racemose. Tepals erect, unequal, the outer three smaller than the inner three; stamens 9, 4-celled, the outer 6 with the cells in a shallow arc, the inner 3 with the cells placed in two pairs, the filaments short, as wide as the anthers, staminodia 3; receptacle deep. Fruit initially enclosed in the cupule, at maturity either 2/ exserted or almost completely enclosed in the cupule. The new genus Sextonia is readily recognized by having clustered leaves and flowers with unequal tepals, characters already mentioned by Rohwer et al. (1991). The differences in wood and bark anatomy between Ocotea and Sextonia (as O. rubra) were amply discussed by Richter (1981), and indeed the main reason that Rohwer et al. (1991) did not describe a new genus to accommodate O. rubra was their reluctance to describe another monotypic genus. In a genus as large as Ocotea it is not surprising to find individual species or groups of species that differ in one or more morphological characters from the rest of the species, and finding such differences is not a sufficiently strong argument for describing a new genus. However, when such morphological differences are supported by differences in wood and bark anatomy the argument for a new genus is stronger. Three reasons made me consider the generic status of 0. rubra again. The first is the recent discovery of an undescribed species closely related to O. rubra, sharing with it the clustered leaves and flowers with unequal tepals, but differing in having pubescent flowers and racemose inflorescences. This new species has wood and bark anatomy similar to O. rubra. Describing this species in Ocotea would clearly be incorrect. The second argument is that during a study of the embryology of Lauraceae (Heo et al., in prep.) Ocotea rubra was found to differ clearly from the other four investigated species of Ocotea in having the mature embryo sac protruding from the nucellus, a character state otherwise only found in Beilschmiedia, Caryodaphnopsis, Cryptocarya, Endiandra, and Potameia, but lacking in genera considered closely related to Ocotea, such as Aiouea, Aniba, Cinnamomum, Licaria, Mezilaurus, and Nectandra. The third is that the inflorescences of the two species of Sextonia are different from those found in Ocotea. In Ocotea the flowers are arranged in cymes, with the lateral flowers of a cyme strictly opposite. In Sextonia rubra, however, the flowers are not arranged in cymes. Frequently the ultimate branching of the inflorescences appears racemose and flowers are not strictly opposite. The inflorescence types of Lauraceae and their importance for the suprageneric classification are discussed in detail by van NovoN 7: 436-439. 1997. This content downloaded from 157.55.39.151 on Thu, 07 Jul 2016 05:31:01 UTC All use subject to http://about.jstor.org/terms Volume 7, Number 4 van der Werff 437 1997 New Genus of Lauraceae