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Asia-Tropical:, Borneo present; Lesser Sunda Is. absent; New Guinea present E. Java: absent Peninsular Thailand: present southern Burma: present
Peninsular Thailand and southern Burma, Malesia: except E. Java and Lesser Sunda Islands (greatest diversity in Borneo), few specimens from New Guinea.


Species of this genus were first recognized as forming a distinct group by Christensen (1929). Ching transferred two of them to Thelypteris in 1936, and the species of Malaya were treated under that genus by Holttum in 1955. In 1963 Ching established the genus Mesoneuron for them, but as Mezonevron Desf. has also been spelled Mesoneuron I proposed the new name Mesophlebion in 1971; in doing so I included Plesioneuron as a subgenus.
Mesophlebion agrees with Cyclosorus (in the strict sense of the present account) and Ampelopteris in the large red glands which occur at the ends of hairs on stalks of sporangia and sometimes on the lower surface of veins, also in the presence of scales on the lower surface of costae. In my judgement, these two genera and Thelypteris are the only near relatives of Mesophlebion.
In tropical America occur two species which Christensen () placed with doubt in Dryopteris subg. Steiropteris; Baker confused Mesophlebion motleyanum with one of these and was copied by other authors. The American species differ from Mesophlebion in having a long sinus-membrane and long aerophores, and in the absence of red glands.
Several species of Mesophlebion are variable, and it is possible that a complex of diploids, triploids and tetraploids may exist. Experimental studies are needed to clarify this Situation. I judge that the species here recognized are reasonably distinct, though probably not all of equal status, and hybrids may occur. Several plants of both M. motleyanum and M. falcatilobum, from Gunong Mulu in Sarawak, have shown consistently distinct characters in cultivation at Kew.
It is notable that, as in Plesioneuron and Chingia, a few species of this genus have abundant scales throughout the stipe and at least the basal part of the rachis. This development in the three genera appears to be a case of parallel evolution; a similar condition occurs in one non-Malesian species of Christella (C. crinipes (Hook.) Holttum) to which Ridley referred specimens of M. trichopodum.


Base chromosome number 36; M. crassifolium and M. trichopodum both tetraploid in Malaya (Manton in Holttum 1955), M. falcatilobum diploid in Sarawak (T. G. Walker, new obs.).


Holttum - in Rev. Fl. Mal. 1955: f. 139-141