Oleandraceae

Primary tabs

Oleandraceae

Distribution

Asia-Tropical:, Borneopresent Australasia: New Guinea and the surrounding islands: present Pacific: present Pantropical: present continental Asia: present
Oleandra is pantropical, but all the individual species are more limited in distribution. Some of the Malesian species extend to Continental Asia, Australia or the Pacific. One is endemic to Borneo, and two are restricted to New Guinea and the surrounding islands.

Morphology

The morphology and anatomy of Oleandra has been studied by various authors (Ogura 1939; Wetter 1951; Phillips & White 1967; Sen & Sen 1973; Nayar & Bajpai 1978), mainly with an emphasis on the structure of the rhizome, of which the climbing habit in some species is a striking character.

Much of the variability between species in Oleandra is found in the rhizome, but as large parts of the rhizome are usually not preserved in herbarium collections, it is difficult to appreciate and describe this variability. In the Malesian species, rhizomes may be relatively short-creeping, leading to more or less compact clumps of fronds, or more widely creeping. In the latter case, in some species the rhizome appears to be short-lived, decaying less than 1 m behind each growing point, which leads to stands with a scattered growth of plants. In others, the rhizome can be highly persistent, and stands may be extensive, with a dense growth of stems. In plants with this growth form, parts of the rhizome may also grow outwards or upwards from the substrate and form erect or drooping stems, here to be called ‘aerial’ stems, that are often rootless. This is the shrubby growth form that is most often associated with the genus, although it occurs only in a minority of species. Branches are often opposite each other, and then both branches follow the same growth, either downwards (positively geotropic) or upwards (negatively geotropic). The anatomy shows a variably, but often very strongly developed peripheral sclerified sheath, a ground tissue with or without scattered sclerenchyma strands, and a dictyostele.

The rhizome is covered with usually persistent rhizome scales with a peltate attachment. The scales are often strongly thickened near the attachment, and the margin can be nearly entire or densely set with woolly hairs (best visible in young scales) or sessile glands. The scales are strongly appressed or spreading to recurved — in the latter case the recurved parts tend to disappear on older rhizomes, giving the impression of a cover of short, appressed scales.

Roots arise mostly from the ventral side of the rhizome, and may be unbranched for a considerable length. The long unbranched parts have been described as rhizophores, but were identified as real roots by Wetter (1951). Branching associated with the formation of root hairs occurs presumably when the roots are in contact with a suitable substrate, and thus the unbranched parts tend to be more evident in species that creep over the substrate, often at some distance, or over dense cushions of moss, than in species with a subterraneous rhizome.

Fronds arise on the rhizome without any apparent regularity, often clustered in more or less distinct whorls or clumps, but on creeping rhizomes often also more widely separated. They do not appear to grow in regular series, but are inserted more or less dorsally on creeping stems, and often on all sides on aerial stems. At a variable position on the stipe, there is a distinct articulation point, where old fronds abscise cleanly. The lamina is uniformly simple in all species, and varies little in shape, except in O. werneri where the fronds are distinctly dimorphic, with the fertile fronds or parts strongly contracted. Sori are indusiate with a reniform indusium, and always separate. They are borne dorsally and singly on the veins, at distances from the costa that may vary strongly. Sporangia are long-stalked, often with a number of glandular hairs attached to the distal part of the stalk, just below the capsule. The capsule is of the common Polypodiales type.

The spores of Oleandra have been studied with SEM by Liew (1977), Tryon & Lugardon (1991) and Hovenkamp & Ho (in press). The morphology of the spores of the Malesian species is highly variable but essentially uniform, and the variation can be described in terms of a restricted number of parameters.

The spore wall is composed of a smooth exospore, and a thick but hollow and often highly ornamented perisopre. The surface of the perisopre shows a pattern of coarse folds, a variable ornamentation and a variable degree of perforation. The folds may be broad, narrow, or replaced by elongated fissures, the ornamentation ranges from warty to densely spinose. This surface may be perforated or perforated to varying degrees, exposing an inner structure of numerous narrow cylindrical pillars c. 0.5 µm thick. These pillars are attached to a thin basal layer that adheres closely to the exospore. In the extreme case, the outer surface is so strongly perforated that the entire perispore consists of an open mesh. Within this range of variability, most species that could be studied with an adequate sample of specimens show variability in 2 or more of these parameters, and it is difficult to identify character states that are characteristic for one species, although it appears that O. sibbaldii has a more consistently highly perforated perispore than the other species.

The size of the spores of a single specimen varies from c. 35-45 to 60-70 µm, also between specimens of one species.

Taxonomy

The genus Oleandra was described by Cavanilles (1799, 1802) who derived both the genus name and the species name from Nerium oleander L. (Apocynaceae). From this it should be clear that he only saw the aerial stems of Oleandra neriiformis, of which the forms with distinctly whorled fronds are indeed strongly reminiscent of branches of Nerium oleander. Oleandra was included in Aspidium by Swartz (1801, 1806), but reinstated by Presl (1836) and John Smith (1841, 1842), and the distinctness of the genus has not been questioned by subsequent authors while only one author has preferred to recognize Oleandra at subgeneric level (Splitgerber 1840).
There is as much unanimity on the distinctness of the genus as there is uncertainty on the number and distinctness of the species that can be distinguished in Oleandra, and many authors who have tried to deal with the genus have commented on the difficulties of species delimitation. Kramer (1990) estimated the number of species to be 40 (“many species are similar in appearance”) and Pichi Sermolli (1965) gives a number of 50 (“the delimitation of the species is not always easy, since some of them show a high degree of polymorphism”)”. Hovenkamp & Ho (in press) distinguish 9 species in Asia, of which 7 occur in Malesia.

Cytology

Chromosome numbers have been established for the Malesian species O. musifolia and O. neriiformis (Löve et al. 1977) but not on basis of Malesian specimens. In both species, the number was diploid with n = 41. The variability in spore size may be an indication of the presence of different ploidy levels (Harmata & Kornás 1978).

Notes

The simple morphology of Oleandra has made comparison with other ferns difficult, and its position has been judged to be with Dryopteridaceae or Davalliaceae, where it has been associated in particular with Nephrolepis and Arthropteris (Copeland 1947; Holttum 1959; Nayar & Bajpai 1978). Holttum (1959) included Oleandra with Nephro­lepis and Arthropteris in an informal Nephrolepis-group. More recent views (Tryon & Tryon 1982; Kramer 1990; Tryon & Lugardon 1991) exclude Nephrolepis, and molecular studies (Kuo et al. 2011; Schuettpelz & Pryer 2007) indicate that these three genera do not form a clade, but that they are each associated with a different clade in the crown leptosporangiates. Here we treat Oleandraceae as a monogeneric family, with the single genus Oleandra.

Citation

K.U.Kramer - in K.U.Kramer & P.S.Green, eds., Fam. Gen. Vasc. Pl. 1990: 190
A.R.Sm. et al. - in Taxon. 2006: 718
Hovenkamp & B.C.Ho - in PhytoKeys. 2012: 1