Considering the soft nature of the body of recent Liverworts, it is not surprising that they are poorly represented in a fossil state. In the absence of the sexual reproductive organs, or of the sporophytes, which have scarcely ever been preserved, exact identification is almost hopeless. The difficulties already referred to in dealing with the algae, as regards the misleading similarity between the form of the thallus and the bodies of other plants, have to be faced in the case of the Liverworts. Many of the thalloid Liverworts, if preserved in the form of a cast or impression without internal structure or reproductive organs, could hardly be distinguished from various genera of algae in which the thallus has the form of a forked plate-like body. Such genera as Pellia, Marchantia, Lunularia, Reboulia, and others bear a striking resemblance to Fucus, Chondrus and many other algae.
Imperfect specimens of certain Lichens, not to mention some of the Polyzoa, might easily be mistaken for Liverworts. Among the higher plants, there are some forms of the Podostemaceae which simulate in habit both thalloid and foliose Liverworts as well as Mosses[450]. The members of this Dicotyledonous family are described as water-plants with a Moss- or Liverwort-like form; they occur on rocks in quickly-flowing water in the tropics. In one instance a recent Podostemaceous genus has been described as a member of the Anthocerotales; the genus Blandowia[451], referred to by Willdenow as a Liverwort, has since been recognised as one of the Podostemaceae. The resemblance between some of the foliose Hepaticae and genera of Mosses is often very close. In certain Mosses, such as Hookeria pennata[452], the large two-ranked leaves suggest the branches of a Selaginella.
Fig. 48. A. Tristichia hypnoides Spreng. From a specimen in the British Museum. B. Podocarpus cupressina Br. and Ben. (After Brown and Bennett[453].) C. Selaginella Oregana Eat. From a plant in the Cambridge Botanic Garden. A, B and C very slightly reduced.
The plant reproduced in fig. 48 A (Tristichia), one of the Podostemaceae, might easily be mistaken for a foliose Liverwort if found as a fragmentary fossil. Such species of Selaginella as S. Oregana Eat. and S. rupestris Spring (fig. 48 C) have a distinctly moss-like habit and do not present a very obvious resemblance to the more typical and better known Selaginellas. The twig of a Podocarpus (P. cupressina)[454] in fig. 48 B affords an instance of a conifer which simulates to some extent certain of the larger-leaved Liverworts; it bears a resemblance also to some fossil fragments referred to Selaginellites or Lycopodites. A small fossil specimen figured by Nathorst[455] from Japan as possibly a Lycopodites may be compared with a coniferous twig, and with some of the larger Liverworts, e.g. species of Plagiochila[456]. Podocarpus cupressina is, however, chiefly instructive as an example of the striking differences which are met with among species of the same genus; it differs considerably from the ordinary species of Podocarpus, and might well be identified as a member of some other group than that of the Coniferae.
We have no records of Palaeozoic Hepaticae. The fossils which Zeiller has figured in his Flore de Brive as Schizopteris dichotoma Gümb.[457] and S. trichomanoides Göpp. bear a resemblance to some forms of hepatics, but there is no satisfactory evidence for removing them from the position assigned to them by the French writer. In Mesozoic rocks a few specimens are known which bear a close resemblance as regards the form of the thalloid body to recent Liverworts, but the identification of such fossils cannot be absolutely trusted. Two French authors, MM. Fliche and Bleicher[458], have described a plant from Lower Oolite rocks near Nancy as a species of Marchantia, M. oolithus, but they point out the close agreement of such forked laminar structures to algae and lichens. From Tertiary and Post-Tertiary beds a certain number of fossil species have been recorded, but they possess no special botanical interest.
Order Marchantiales.
The plant-body is always thalloid, bearing rhizoids on the lower surface, and having an epidermis with pores limiting the upper or dorsal surface.
Marchantites.
This convenient generic name was proposed by Brongniart in 1849[459]; it may be briefly defined as follows: