Pleocnemia

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Pleocnemia

Description

thick glandular hairs, spherical, clavate or cylindrical, yellow or red, present at the ends of hairs on the stalks of sporangia, often also on the lower surface of costules and veins.

Distribution

Malesian, extending to NE India and SE China, eastwards to Samoa: present
Mainly Malesian, extending to NE India and SE China, eastwards to Samoa; about 20 species.

Taxonomy

There are two groups of species, for the types of which Presl in 1836 established the genera Pleocnemia and Dictyopteris (later renamed Arcypteris Underw.). I retained both genera in a monographic treatment in 1951 but remarked on their close alliance, and united them in 1974. A probable hybrid having one parent from each group is known (P. × intermedia). The only differences between the two groups are a more complex venation and less distinct sinus-teeth in Arcypteris.

Confusion as regards generic assignment of these ferns in the 19th century resulted from emphasis on two kinds of characters: pattern of venation, which these ferns share with Tectaria sect. Sagenia (and less clearly with some thelypteroid ferns), and the presence or absence of indusia. Until Diels (1899) it was not fully understood that species lacking indusia could be closely allied to others which have them and thus be congeneric.

Furthermore, the distinctive combination in Pleocnemia of some other characters was ignored. To anyone seeing a living plant, as did Gaudichaud when he collected specimens of P. leuzeana, the presence of sinus-teeth which project out of the plane of a pinnule, described and figured by him, is striking. The sinus-teeth were noticed by Gaudichaud, John Smith (who had living plants in his care) and for one species by Van Alderwerelt; Presl, Fée and Hooker ignored them and omitted them from their illustrations. Unicellular yellow or orange glands were noticed by various authors but not thought to be of generic significance. The peculiar vascular anatomy of stipes was observed only by Fée for P. macrodonta and shown in a distorted form from a dried specimen.

The species here placed as nos 1-4 are closely allied; two of them have been ranked as varieties of P. irregularis. They agree in exindusiate sori, for which reason earlier authors placed them in Polypodium, or in Dictyopteris which Presl placed with polypodioid ferns. The original species of Pleocnemia is also exindusiate and was earlier named Polypodium leuzeanum by Gaudichaud; in 1836 Presl placed it with polypodioid ferns. But among Cuming's Philippine collections were four numbers, all resembling P. leuzeana in frond-form and venation, two of them clearly indusiate. John Smith, who made the first survey of Cuming's collections (1841) regarded all four as forms of P. leuzeana, which he placed with aspidioid ferns, remarking that indusia in these ferns were often small and soon shed. Hooker accepted John Smith's judgement and illustrated a fertile pinnule of p, cumingiana and a sterile one of P. conjugata under the name P. leuzeana. Later Beddome (1866) published as P. leuzeana a drawing of the sterile apex of a frond (probably P. conjugata) from Moulmein in Burma along with an enlarged drawing of a fertile pinnule-lobe of P. cumingiana, not stating that the latter was from a non-Burmese specimen (he cited P. conjugata and P. cumingiana as synonyms).

In 1857 Thomas Moore accepted the genus Pleocnemia, defining it by frond-form, venation and indusiate sori, and added one species of Heterogonium and one thelypteroid species. In 1864 Hooker, ranking Pleocnemia as a section of Nephrodium, added two more thelypteroid species, but John Smith (1875) rejected these additions. Beddome (1883) added species here included in Tectaria sect. Sagenia and he placed all exindusiate species of Tectaria in Dictyopteris. In general, Van Alderwerelt followed Beddome. Copeland, in all his later works, included the type species of both Pleocnemia and Dictyopteris Presl in Tectaria.

Though Copeland lived for several years at Los Banos, at the foot of Mt Makiling in Luzon, he never discovered distinctions among the plants of Pleocnemia s.s. growing in the forest on that mountain and in 1960 included all in P. leuzeana. He did recognize as distinct an exindusiate species in Sumatra (P. olivacea) in 1914 and Van Alderwerelt another in Java (P. hemiteliiformis). From my field and herbarium studies based on Singapore I recognized that Pleocnemia is a genus distinct from Tectaria and that (apart from P. irregularis) three distinct species exist in the Malay Peninsula. After a further study of specimens in the Bogor herbarium I identified the peninsular species as P. olivacea, P. hemiteliiformis and P. conjugata (Blume) Presl, the last-named based on Aspidium conjugatum Blume (type from Java), a name ignored by Backer and Posthumus in their fern- flora of Java. I also saw duplicates of all the Cuming Philippine collections and realized that they do not all belong to one species. I attempted to account for all this new information in a paper published in 1951. In more recent years M. G. Price has made detailed field studies of Philippine species and has confirmed the distinctions between species which I had based on Cuming's specimens; this further information is included in a second paper, published in the Kew Bulletin in 1974.

The species of Pleocnemia s.s. (nos 6-18) are all similar in frond-form and venation. The fronds of mature plants of all are very large and cannot be mounted whole on herbarium sheets. The basal pinnae are of distinctive form on the basiscopic side near the base, but distally are much like those next above them; more distal pinnae decrease gradually. Some herbarium specimens are from immature plants; few are annotated to indicate which part of a frond they represent. Thus it is sometimes difficult to make comparisons, between one specimen and another. For purposes of identification one must rely mainly on the form of pinnules of larger suprabasal pinnae and on the sori they bear. The scales at the bases of stipes are also sometimes important but are lacking from most specimens. The abundance of appressed glands on the lower surface of veins appears to be variable in fronds of the same species, also their thickness; their shape and colour may be distinctive. More evidence from living plants is needed.

In 1912, dealing with specimens from New Guinea, Rosenstock remarked on the differences in spore-form from specimens which he regarded as different varieties of P. leuzana. I examined spores of all species and confirmed that they are of three kinds, basing my key of 1951 on them. They offer important evidence for the distinctions between P. cumingiana and P. presliana which otherwise are not easy to distinguish except that the former is indusiate, the latter not. Field study has shown that plants of P. presliana are smaller and have different scales. Cytological examination might provide further evidence and experimental hybridization might give further understanding.

Cytology

n = 41 .

Citation

Hook., Gen. Fil. 1842: t. 70A, f. 3 only, & t. 97
J. Sm., Ferns Brit. For. 1866: f. 71
Bedd., Handb. Ferns Brit. India. 1883: 223
Presl - in Kew Bull. 1974: 341
Alderw., Malayan Ferns. 1908: 170, 810
Holttum - in Reinwardtia. 1951: 191
Fée - in Mém. Foug. 1852: 267
Presl, Epim. Bot. 1851: 50
Holttum - in Reinwardtia. 1951: 171
Fée - in Mém. Foug. 1852: 311