Before proceeding to the classification of those plants which form a more conspicuous component of the Earth’s flora, mention must be made of a group which has a long geological history, but which has remained much in the background.
This group is known as the Bryophyta, the mosses and liverworts. There seems little doubt that these two sub-groups evolved, probably independently, from the Chlorophyta with which they have a number of characters in common.
However, their life-cycle is not well suited to terrestrial life. The gametophyte generation is the dominant one and the smaller sporophytes grow upon it. In many cases, particularly in the liverworts, the sporophytes have no capacity to carry out photosynthesis. Thus, the group is in a situation where the dominant generation is poorly adapted to terrestrial life and unable to compete successfully with those plants having dominant sporophytes. With such a fundamental disadvantage, the bryophytes have not given rise to increasingly evolved forms as have the other major plant groups. The gametophyte plants in some liverwort species are thalloid. That is to say they have the shape of pieces of brown seaweed.
Some liverworts and most mosses are described as ‘leafy’, but their structure is not one in which there is a very clear division into stem, leaf, and root. Indeed, they have no true roots and their leaves grow more or less directly out of their stems, thin, flat plates one cell thick. There are elongate cells in the centre of their stems and down the midribs of leaves in some species which may help in moving solutions up to, or down from the leaves. They are clearly extremely primitive plants which can only grow in restricted habitats on land. It is of interest though, that the sporophytes possess stomata which are features seen in all higher plants.
The earliest fossil plant that has a claim to be considered a bryophyte is Sporogonites from about 400 million years ago. There is some doubt about the affinities of this plant, but this is not true of allavicinites which is an undoubted bryophyte from about 370 million years. It is obvious therefore that the group has a long history and representatives become increasingly common in more recent geological times. In fact, the ones that lived more recently can be more or less directly related to living genera.
Fossil examples: Musites, Ditrichites, Dierttertia, Aulacomnium, Protosphagnum.
Marchantiolites, Naiadita, Hepacitites, Thallites, Ricciopsis, Marchantites.