Heterogonium

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Heterogonium

Distribution

Asia-Tropical: Burma to South China: present Mauritius: present
Burma to South China; Malesia; Mauritius; 22 known species.

Taxonomy

In the 19th century the species here brought together were very variously treated by different authors. Specimens of the type species were assigned to the genera Stenosemia, Gymnogramma, Digrammaria, Phegopteris and Aspidium, of other species to Nephrodium, Dryopteris, Lastrea, Polypodium, Acrostichum, Polybotrya and Pleocnemia. As here construed (following Holttum 1949 and 1975) Heterogonium might rank as a subgenus of Tectaria. As in Tectaria, it comprises species with free and anastomosing veins, but differs constantly in the shape of basal pinnae and in the venation of species which have broad pinnae, also in a more frequent tendency to the exindusiate condition with sori more or less spreading along veins, and to an apparently acrostichoid condition where the fertile pinnae are greatly contracted.

Presl’s type species has the venation of Sagenia Presl with exindusiate sori spreading a variable distance along the veins. The genus was ignored by other authors until Copeland added another species in 1929. Copeland failed to notice the form of basal pinnae. Christen- sen added two more species in 1934; these are here united as H. stenosemioides which, having free veins, does not correspond to Copeland’s definition of 1947 and was ignored by him.

In 1938 Ching made the first attempt to deal with Ctenitis in Asia. In the New World Christensen had found that Tectaria and Ctenitis agree in having hairs of a peculiar type (a character ignored in the 19th century) and remarked that they were distinct in the anastomosing veins of Tectaria and the free ones of Ctenitis. But in Asia Ching found that there were free-veined species which agreed with Tectaria, not Ctenitis, in some details of venation, and proposed for them the new genus Ctenitopsis. In it he included six species which have basal pinnae as in Tectaria and two with basal pinnae of the Heterogonium form here recognized; all those published before 1905 had been included in Dryopteris in Christensen’s Index Filicum. Copeland (1947) included Ctenitopsis in Ctenitus, without comment on basal pinnae.

Holttum (1949 and 1975) contended that the recognition of the form of basal pinnae as a distinctive character separated Heterogonium as a natural group from Tectaria when all relevant species are considered. But there is one known case of apparent hybridization between a species having basal pinnae as in Tectaria (T. aurita) with one having basal pinnae as in Heterogonium (H. pinnatum). Experimental study of the relationships between these and other species of the two genera is needed.

Cytology

Chromosome number n = 41.

Citation

Holttum - in Sarawak Museum J. 1949: 155
Presl - in Kalikasan. 1975: 205
Copel, Gen. Fil. 1947: 127