Plesioneuron

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Plesioneuron

Distribution

Amboina: present Asia-Tropical:, Bismarck Archipelago (Bismarck Archipelago present); Borneo present; Maluku (Maluku present); New Guinea present Batjan: present Buru: present Mindanao: present Pacific: present Tahiti: present Tidore: present islands of the Pacific: present
Malesia&Pacific: Borneo (2 spp.); Moluccas (Tidore, Batjan, Buru, Amboina, 2 spp.); Mindanao (1 sp.); New Guinea&Bismarck Archipelago (32 spp.); islands of the Pacific, to Tahiti (12 spp.).

Taxonomy

The name Plesioneuron refers to the basal basiscopic veins in each pinna-lobe which always arise close to their costules, never far from them as in Mesophlebion. Plants of many species are similar in general aspect, and to some extent in venation, to Mesophlebion, to which genus they were assigned as a subgenus in 1971, but they differ from Mesophlebion in sporangia and spores, also, in a majority of species, in scales, and (in the only case investigated) in gametophytes. They appear to be related to Chingia and not very closely to any other genus. Copeland included them in Lastrea, and published a KEY TO THE SPECIES of that genus in New Guinea () but he did not characterize them clearly nor distinguish them as a group. There are undoubtedly many species in New Guinea, but specimens of some are few or imperfect, so that more information about them is needed, and the present account probably needs some modification. Some type specimens may represent immature plants (an example is P. ophiura) and more local collectihg is needed to establish this; in other cases small plants probably do represent the mature form of a species. Recent collections certainly include plants of previously undescribed species, and probably more remain to be discovered.
Several species show some degree of difference between lobes on acroscopic and basiscopic sides of a pinna, but there is no sharp distinction between these and those with no clear difference. However, in a few cases the difference is very marked and can be used in the key to the identification of species.
The sinus-membrane in Plesioneuron is in all cases distinctly decurrent from the base of the sinus as a more or less hairy ridge on the lower surface, sometimes almost reaching the costa. This condition is closely similar to that of Dryopteris subg. Steiropteris C. Chr. (), a group of species in the American tropics. These species differ from Plesioneuron as follows: rhizome wide-creeping; septate hairs on lower surface of rachis and costae; neither glands nor setae on sporangia; spores with translucent wing and a few cross-wings (this type of spore occurs in a few species of Plesioneuron). Two aberrant species included with doubt by Christensen in subg. Steiropteris are more similar to Mesophlebion and are mentioned under that genus.
I believe that characters of scales are often of diagnostic significance, but in many specimens they are not well represented; more information about them may help to clarify specific distinctions.

Cytology

Chromosome number 36 (P. fulgens only).

Citation

Holttum - in Allertonia. 1977: 186
L. R. Atkinson - in Phytomorphology. 1975: 45