Gleichenia subg. Mertensia
Notes
This subgenus includes far more species than any other major division of the family, in all continents, and its distribution is mainly south of the equator. Most of the species are rather small in size, as compared with representatives of subg. Diplopterygium, and could not compete as thicket- forming ferns either with the latter or with Dicranopteris. The only Malaysian species which can so compete is G. truncata, and it is also the only species of subg. Mertensia which occurs at sea-level in our region. The others are mountain plants occurring in scrub and dwarf forest of high ridges or in open grassy or rocky places, which are comparable with the habitats of related species in south temperate regions.
As in other divisions of the family, the characters and distribution of scales and hairs are important diagnostically in this subgenus. The number of times the lateral branch-systems are forked (the number of orders of forking) is probably important, but shows considerable Variation within a species according to the age of the plant and to environmental conditions, and these cannot be fully judged from dried specimens. A more important kind of character is the relative length of branches of the first and ultimate orders. But in some species one frond will have branch-systems forked to 2 orders with long ultimate branches, other fronds (even part of the same frond) may be forked to 3 orders with much shorter ultimate branches. The glaucous character of the lower surface of the lamina may be significant, but is easily destroyed by heat in drying. In all these characters, and in all species, more field study is needed to establish distinctions between species more clearly.
Judging from anatomical characters, Chrysler () has argued that subg. Mertensia is the most primitive division of the genus. But from the point of view of sporangia and sori it is exactly in equality with subg. Diplopterygium, and in leaf-form it is clearly more highly or- ganized (in having a branching-pattern dependent on a series of permanently dormant apices). It seems to me significant that among fossils none are clearly referable to this subgenus, whereas there are abundant fossils having a close resemblance to subg. Diplopterygium. An apparent exception is Gleichendes gracilis Zigno. But the branching-pattern of this fossil is very irregular and there is no clear evidence of pseudo-dichotomy; I think that its resemblance to subg. Mertensia is superficial.
As in other divisions of the family, the characters and distribution of scales and hairs are important diagnostically in this subgenus. The number of times the lateral branch-systems are forked (the number of orders of forking) is probably important, but shows considerable Variation within a species according to the age of the plant and to environmental conditions, and these cannot be fully judged from dried specimens. A more important kind of character is the relative length of branches of the first and ultimate orders. But in some species one frond will have branch-systems forked to 2 orders with long ultimate branches, other fronds (even part of the same frond) may be forked to 3 orders with much shorter ultimate branches. The glaucous character of the lower surface of the lamina may be significant, but is easily destroyed by heat in drying. In all these characters, and in all species, more field study is needed to establish distinctions between species more clearly.
Judging from anatomical characters, Chrysler () has argued that subg. Mertensia is the most primitive division of the genus. But from the point of view of sporangia and sori it is exactly in equality with subg. Diplopterygium, and in leaf-form it is clearly more highly or- ganized (in having a branching-pattern dependent on a series of permanently dormant apices). It seems to me significant that among fossils none are clearly referable to this subgenus, whereas there are abundant fossils having a close resemblance to subg. Diplopterygium. An apparent exception is Gleichendes gracilis Zigno. But the branching-pattern of this fossil is very irregular and there is no clear evidence of pseudo-dichotomy; I think that its resemblance to subg. Mertensia is superficial.