Salviniaceae.
The two genera of Salviniaceae, Salvinia and Azolla, are water plants, and are usually described as annuals which survive the less favourable season in the form of detached sporocarps. Goebel[1211] states that all the tropical species of Salvinia known to him have an unlimited existence.
Salvinia natans, Hoffm., the only European species, extends from the South of France to Northern China and the plains of India: the other twelve species are mostly tropical. Azolla, represented by four species, occurs in Western and Southern North America, South America, Madagascar, Australia, New Zealand, and is widely spread in tropical Asia and Africa.
Species of Azolla frequently form a considerable proportion of the floating carpet of vegetation on inland waters[1212] growing under conditions which might be supposed favourable for preservation in a fossil state.
The Salviniaceae, though probably rather farther removed than the Marsiliaceae from the homosporous Filicineae, are considered by Bower[1213] to be related to the Gradatae, but modified in consequence of their aquatic habit and the assumption of heterospory.
No undoubted examples of fossil species of Azolla have been described. Salvinia, on the other hand, is represented by several Tertiary species, for the most part founded on leaves only, and Hollick[1214], who published a list of fossil Salvinias, has described detached leaves as Salvinia elliptica Newb. from what may be Upper Cretaceous rocks from Carbonado, Washington. Some of the leaves figured as Tertiary Salvinias are of no value as evidence of the former distribution of the genus[1215].
From the Coal-beds of Yen-Bäi (Tonkin), probably of Miocene age, Zeiller[1216] has figured some well-preserved impressions of oval or orbicular leaves, 15 mm. long and 10–20 mm. broad, characterised by reticulate venation and by cordate bases, which he refers to Heer’s Swiss species Salvinia formosa[1217].
Dr Zeiller[1218] in the most recently published part of his series of valuable résumés of palaeobotanical literature refers to a description by Brabenec of specimens of this species from Bohemian Tertiary beds showing both microspores and megaspores.
One of the most complete specimens so far discovered has recently been described by Fritel[1219] from Eocene beds of the Paris Basin as Salvinia Zeilleri. This species, founded on portions of stems bearing floating leaves, submerged root-like leaves, and sporocarps, is compared with a recent tropical American species S. auriculata.
It is noteworthy that no authentic records of Hydropterideae have been discovered in Palaeozoic rocks[1220]. Comparisons have been made in the case of the genera Traquairia Carr. and Sporocarpon Will. with the reproductive organs of Azolla[1221], but these rest on a wholly insufficient basis.
Dawson[1222] proposed the generic name Protosalvinia for some spores of Devonian age, which he regarded on inadequate grounds as evidence of Palaeozoic Hydropterideae.
Zeiller[1223], in discussing the possible relationships of the problematical type Chorionopteris gleichenioides Cord., suggests a possible alliance with the Hydropterideae. Corda founded the genus Chorionopteris[1224] on some small fragments of pinnules, 6–7 mm. long, found in the Carboniferous rocks of Radnitz in Bohemia.
The lobes of the pinnules are incurved distally to form a capsule, containing four sporangia, which apparently opened on dehiscence into four valves; the spores are of one size. The material is however insufficient for accurate determination.
There is no evidence contributed by fossil records which indicates a high antiquity for the Hydropterideae. It is unsafe to base any conclusion on the absence of undoubted Palaeozoic representatives of this group; but the almost complete absence of records in pre-Tertiary strata is a fact which may be allowed some weight in regard to the possible evolution of the heterosporous filicales at a comparatively late period in the earth’s history.
A description of the Mesozoic genus Sagenopteris may be conveniently included in this chapter, though as in many other instances the inclusion of a genus under the heading of a recent family name does not by any means imply that the position of the extinct type is regarded as settled.