Pronephrium

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Pronephrium

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Taxonomy

The following species, included here in 1972, are now transferred to other sections: the non-Malesian P. lakhimpurense and allied species, also the Malesian P. nitidum, P. repandum and P. acanthocarpum, to sect. Menisciopsis; P. aquatiloides and P. menisciicarpon to sect. Dimorphopteris.

The species here included belong to three groups:
  1. Species which appear to be related to the type, P. lineatum. Of these, P. euryphyllum has glands on the lower surface and appressed hairs on the upper surface as in P. glandulosum of sect. Dimorphopteris. P. asperum and P. gymnopteridifrons lack glands but have the same frond-form as the type. None of these have the reddish tinge shown by species of subg. Menisciopsis. But they show resemblances to Sphaerostephanos peltochlamys, S. simplieifolius and S. spenceri, which differ only in the presence of several pairs of much-reduced basal pinnae.
  2. Species which have an elongate apical lamina and a few much smaller pinnae, and sori which lack indusia. These are specialized forms the relationship of which it is difficult to suggest. Three of them have glands on their sporangia as in P. lineatum. The most aberrant species, lacking glands, is P. micropinnatum; it may belong elsewhere.
  3. Species included in Haplodictyum in my arrangement of the genera in 1971, but excluding H. majus Copel. which is now placed in Sphaerostephanos because it has much-reduced basal pinnae in addition to normal ones. These species have glands on the lower surface of pinnae and indusiate sori.

Two of them have the peculiar venation in their terminal lamina which caused Presl to establish the genus Haplodictyum: the veins in the lower part of the apical lamina are forked, their branches anastomosing to form a series of areoles along the costules, also sometimes additional areoles below the sinuses. This venation resembles that of Pleocnemia, and on that account Ching placed Haplodictyum in his family Aspidiaceae, though it is unquestionably thelypteroid. But the third species (P. bakeri) does not have the veins thus forked, though in every other character it resembles the other two. The Haplodictyum type of venation occurs to a small extent at the base of the apical lamina in several species of other genera of Thelypteridaceae where the transition from apical lamina to pinnae is not sharply defined. The peculiar venation is like that of pinnae, of which the upper ones are more or less adherent to the base of the apical lamina. There is a North American fossil named Goniopteris claiborniana Berry () which shows the Haplodictyum venation.