Aporum

Aporum leonis, from MillFa Wild Orchid.

Belongs within: Dendrobium.

Aporum is a genus of orchids with fleshy, equitant leaves found in south-east Asia and Malesia (Clements 2003).

Succulent orchids
Published 9 April 2022

With over 1200 known species found in Asia and Australasia, Dendrobium is one of the largest currently recognised genera of orchids. As with other examples of such ‘super-genera’, the question of how to best handle such a monster has been fiercely debated. In 2003, Australian botanist M. Clements proposed dividing Dendrobium between numerous segregate genera, noting (among other reasons) that the genus as previously recognised was not monophyletic. However, Clements’ system does not seem to have garnered widespread usage with other orchid systematists preferring to retain a broad concept of Dendrobium (excluding some of the more egregious outliers) that largely corresponds with its established usage (e.g. Schuiteman 2011). Nevertheless, many of the subdivisions promoted by Clements remain recognised as well delimited groups. One such cluster is the assemblage of species recognised as Dendrobium section Aporum.

Growth habit of Dendrobium sect. Aporum, copyright Tony Rodd.

Species of section Aporum are epiphytes found in lowland forests of south-east Asia, extending eastwards to New Guinea and the Solomon Islands. Members of this section have thin stems that are erect at first but tend to become pendulous as they lengthen. Leaves are fleshy and equitant: that is, they are folded longitudinally with what would otherwise be the two sides of the dorsal surface fused, except at the base where they overlap with opposing leaves. The stem may be more or less completely concealed by the leaf bases. Tips of the leaves end in a point. Flowers are borne singly or in clusters, arising laterally on the stem between leaf nodes or at the tip of the stem alongside a terminal scale. The flowers may be subtended by persistent chaffy bracts. They are generally small and fleshy and tend to be short-lived, wilting after just a few days.

Flowers of Dendrobium anceps, copyright Aqiao HQ.

The functional significance of the Aporum section’s distinctive leaves remains uncertain. As noted by Carlsward et al. (1997), the fleshy leaves might be taken as an adaptation to water retention. However, though access to water is a consistent concern for epiphytes, the humid rainforests in which Aporum species are found hardly seem the driest of places. Conversely, the effective even distribution of stomata on both sides of leaf resulting from their equitant condition may make it easier for excess water to be released from the plant.

Dendrobium distichum, photographed by Ronny Boos.

Orchids in general are, of course, most often considered by people as ornamental plants. My impression is that the various Aporum species tend not to be among the most widely grown of species though their unusual growth habit might attract interest. This may be due to them not being the easiest of orchids to maintain; they appear to require high humidity and warm temperatures to thrive with a cooler, drier period in the non-growing season. Among the more popular species are Dendrobium anceps and D. keithii, both of which produce small greenish flowers. Those of D. anceps have been described as having a distinct “apple pie” fragrance. Of course, if you happen to be wandering through the jungles of south-east Asia, you might well discover these plants growing of their own accord.

Systematics of Aporum

Characters (from Clements 2003): Erect, porrect or pendulous, small to large, epiphytic or lithophytic, herbs. Roots thin wiry, divided. Stems thin, wiry, covered with lead sheaths for most of their length but exposed in most species towards the apex, often branching. Leaves equitant, hard, fleshy, broadest near the basal half of each stem, then tapering towards the apex, with a distinct abscission layer, apices acute, persistent for several years, those nearest the apex becoming increasingly smaller and bract like. Inflorescence lateral on exposed apical part of stem, the bract from successive flowering forming a tuft at each node, comprising one or a few flowers. Flowers solitary arising from a lateral meristematic persistent inflorescence, lasting only a few days, synchronous with other plants of the same species in the immediate vicinity; fleshy. Lateral sepals connate at base and spreading towards the apices, together with the labellum and column-foot forming a small spur or saccate base. Dorsal sepal free often much smaller than the lateral sepals. Petals free and often smaller than the dorsal sepal. Labellum fleshy, sessile, rigid with distinct thickened callus of small glands on the laminar surface. Column short with truncate apex, and prominent staminodes. Pollinia small, hard waxy, yellow in two globose hemipollinia. Protocorm-seedling type narrowly isobilateral.

<==Aporum Blume 1825 (see below for synonymy)C03
|--A. leonis Lindley in Edwards 1840 [=Dendrobium leonis]C03
`--+--A. confusum (Schlechter) Clements 2003 [=Dendrobium confusum Schlechter 1911]C03
`--+--A. aloifolium (Blume) Brieger 1981 [=*Macrostomium aloifolium Blume 1825, Dendrobium aloifolium]C03
`--+--A. anceps (Swartz) Lindley 1830 [=Dendrobium anceps, *Ditulima anceps Rafinesque 1838]C03
`--A. indivisum Blume 1825 [=Dendrobium indivisum]C03

Aporum incertae sedis:
*A. lobatum Blume 1825C03
A. acinaciforme (Roxburgh) Griffiths 1845C03
A. albayense (Ames) Clements 2003 [=Dendrobium albayense Ames 1912]C03
A. auyongii (Yukawa) Clements 2003 [=Dendrobium auyongii Yukawa 1998]C03
A. babiense (Smith) Rauschert 1983C03
A. banaense (Gagnepain) Rauschert 1983C03
A. basilanense (Ames) Clements 2003 [=Dendrobium basilanense Ames 1912]C03
A. bicornutum (Schlechter) Rauschert 1983C03
A. bilobulatum (Seidenf.) Clements 2003 [=Dendrobium bilobulatum Seidenf. 1985]C03
A. brevimentum (Seidenf.) Clements 2003 [=Dendrobium brevimentum Seidenf. 1985]C03
A. calceolariae (König) Clements 2003 (see below for synonymy)C03
A. capitellatoides (Smith) Rauschert 1983C03
A. capitellatum (Smith) Rauschert 1983C03
A. cochinchinense (Ridl.) Rauschert 1983C03
A. compressistylum (Smith) Rauschert 1983C03
A. concavum (Smith) Rauschert 1983C03
A. crucilabre (Smith) Rauschert 1983C03
A. curviflorum (Rolfe) Clements 2003 [=Dendrobium curviflorum Rolfe 1805]C03
A. dalatense (Gagnepain) Rauschert 1983C03
A. diaphanum (Schlechter) Rauschert 1983C03
A. distichum (Presl.) Rauschert 1983 [=*Schismoceras disticha Presl 1827]C03
A. escritorii (Ames) Clements 2003 [=Dendrobium escritorii Ames 1915]C03
A. ferdinandi (Kraenzlin) Clements 2003 [=Dendrobium ferdinandi Kraenzlin in Engl. 1910]C03
A. flexile (Ridl.) Rauschert 1983C03
A. fuscum (O’Byrne) Clements 2003 [=Dendrobium indivisum var. fuscum O’Byrne 1997]C03
A. grande (Hooker) Rauschert 1983C03
A. jenkinsii Griffiths 1854 [incl. Dendrobium parciflroum Reichenb. ex Lindley 1859]C03
A. jennae (O’Byrne) Clements 2003 [=Dendrobium jennae O’Byrne 1996]C03
A. keithii (Ridl.) Clements 2003 [=Dendrobium keithii Ridl. 1896]C03
A. kentrophyllum (Hooker) Brieger 1981C03
A. kiauense (Ames & Schweinfurth) Clements 2003 [=Dendrobium kiauense Ames & Schweinfurth 1920]C03
A. kjellbergii (Smith) Rauschert 1983C03
A. korthalsii (Smith) Rauschert 1983C03
A. kuyperi (Smith) Rauschert 1983C03
A. litoreum (Bailey) Clements 2003 [=Dendrobium litoreum Bailey 1906]C03
A. lobbii Clements 2003 [=Dendrobium lobbii Lindley 1859 non Teijsm. & Binn. 1853]C03
A. lobulatum (Rolfe ex Smith) Brieger 1981C03
A. lunatum (Lindley) Clements 2003 [=Dendrobium lunatum Lindley 1859]C03
A. macgregorii Clements 2003 (see below for synonymy)C03
A. mannii (Ridl.) Rauschert 1983C03
A. marivelense (Ames) Rauschert 1983C03
A. merrillii (Ames) Clements 2003 [=Dendrobium merrillii Ames 1908]C03
A. mindanaense (Ames) Clements 2003 [=Dendrobium mindanaense Ames 1914]C03
A. mirandum (Schlechter) Rauschert 1983C03
A. modestissimum (Kraenzlin) Clements 2003 [=Dendrobium modestissimum Kraenzlin in Engl. 1910]C03
A. nathanielis (Reichenb.) Clements 2003 [=Dendrobium nathanielis Reichenb. in Schill. 1857]C03
A. nycteridoglossum (Reichenb.) Rauschert 1983C03
A. pendulicaule (Hayata) Rauschert 1983C03
A. porphyrophyllum (Guillaumin) Clements 2003 [=Dendrobium porphyrophyllum Guillaumin 1955]C03
A. prostratum (Ridl.) Rauschert 1983C03
A. pseudoaloifolium (Wood) Clements 2003 [=Dendrobium pseudoaloifolium Wood 1984]C03
A. pseudoequitans (Fessel & Lückel) Clements 2003 (see below for synonymy)C03
A. quadrilobatum (Carr) Rauschert 1983C03
A. ramificans (Smith) Rauschert 1983C03
A. reflexibarbatulum (Smith) Clements 2003 [=Dendrobium reflexibarbatulum Smith 1927]C03
A. reflexitepalum (Smith) Rauschert 1983C03
A. rhodostele (Ridl.) Rauschert 1983C03
A. rhombopetalum (Kraenzlin) Rauschert 1983C03
A. rosellum (Ridl.) Rauschert 1983C03
A. roseonervatum (Schlechter) Rauschert 1983C03
A. roseostriatum (Ridl.) Rauschert 1983C03
A. sagittatum (Smith) Rauschert 1983C03
A. salicornioides (Teijsm. & Binn.) Brieger in Schlechter 1981C03
A. sambasanum (Smith) Clements 2003 [=Dendrobium sambasanum Smith 1909]C03
A. sarcostomumT-W89
A. shompenii (Sinha & Rao) Clements 2003 [=Dendrobium shompenii Sinha & Rao 1998]C03
A. sinuosum (Ames) Clements 2003 [=Dendrobium sinuosum Ames 1922]C03
A. smithianum (Schlechter) Rauschert 1983C03
A. spatella (Reichenb.) Clements 2003 [=Dendrobium spatella Reichenb. 1865]C03
A. sphenochilum (von Mueller & Kraenzlin) Clements 2003 (see below for synonymy)C03
A. subpandifolium (Smith) Rauschert 1983C03
A. subulatoides (Schlechter) Rauschert 1983C03
A. subulatum (Blume) Rauschert 1983C03
A. teloense (Smith) Rauschert 1983C03
A. teres Rauschert 1983C03 [=Dendrobium teresT-W89]
A. terminale (Par. & Reichenb.) Clements 2003 [=Dendrobium terminale Par. & Reichenb. 1874]C03
A. tetralobum (Schlechter) Rauschert 1983C03
A. thysanophorum (Schlechter) Rauschert 1983C03
A. uncatum (Lindley) Brieger in Schlechter 1981C03
A. vanhulstijnii (Smith) Rauschert 1983C03
A. wenzelii (Ames) Clements 2003 [=Dendrobium wenzelii Ames 1915]C03
A. xanthoacron (Schlechter) Clements 2003 [=Dendrobium xanthoacron Schlechter 1906]C03
A. xiphophyllum (Schlechter) Clements 2003 [=Dendrobium xiphophyllum Schlechter 1911]C03

Aporum Blume 1825 [=Dendrobium sect. Aporum (Blume) Lindl. in Paxt. 1851; incl. Ditulima Rafinesque 1836, Macrostomium Blume 1825, Aporum sect. Macrostomium (Blume) Bieger 1981, Schismoceras Presl 1827]C03

Aporum calceolariae (König) Clements 2003 [=Epidendrum calceolariae König 1791; incl. Dendrobium acerosum Lindley 1841]C03

Aporum macgregorii Clements 2003 [=Dendrobium macgregorii Ames 1912 non von Mueller & Kraenzlin 1894, D. quisumbingii Hawkes & Heller 1957]C03

Aporum pseudoequitans (Fessel & Lückel) Clements 2003 [=Dendrobium pseudoequitans Fessel & Lückel 2000, Ceraia pseudoequitans (Fessel & Lückel) Clements 2003]C03

Aporum sphenochilum (von Mueller & Kraenzlin) Clements 2003 [=Dendrobium sphenochilum von Mueller & Kraenzlin 1894]C03

*Type species of generic name indicated

References

Carlsward, B. S., W. L. Stern, W. S. Judd & T. W. Lucansky. 1997. Comparative leaf anatomy and systematics in Dendrobium, sections Aporum and Rhizobium (Orchidaceae). International Journal of Plant Sciences 158 (3): 332–342.

[C03] Clements, M. A. 2003. Molecular phylogenetic systematics in the Dendrobiinae (Orchidaceae), with emphasis on Dendrobium section Pedilonum. Telopea 10 (1): 247–298.

Schuiteman, A. 2011. Dendrobium (Orchidaceae): to split or not to split? Gardens’ Bulletin Singapore 63 (1–2): 245–257.

[T-W89] Tenison-Woods, J. E. 1889. On the vegetation of Malaysia. Proceedings of the Linnean Society of New South Wales, series 2, 4 (1): 9–106, pls 1–9.

Leave a comment

Your email address will not be published. Required fields are marked *