The Bryophyta are small plants, varying in size from 1 mm. to about 30 cm., creeping or erect, having a thalloid, or more usually a foliose body, consisting of a cell-mass exhibiting in most cases a distinct internal differentiation. They possess no true roots and no true vascular tissue. The life-history of the members of the group is characterised by a well-marked and definite alternation of generations. The Moss or Liverwort plant is the sexual generation (_gametophyte_), and as a result of the fertilisation of an egg-cell the asexual or spore-bearing generation (_sporophyte_) is produced. The sporophyte never exhibits a differentiation into stem and leaves. Asexual and vegetative reproduction are effected by means of spores, bulbils, or detached portions of the plant-body. Sexual reproduction is by means of biciliate antherozoids produced in _antheridia_ and egg-cells formed singly in _archegonia_. In the Bryophytes the distinguishing characteristics are more constant and well-defined than in the Thallophytes. In the former the plant never consists of a single cell or coenocyte, but is always multicellular, and exhibits in most cases a definite physiological division of labour as expressed in the histological differentiation of distinct tissue-systems. In the Thallophytes there is no true alternation of generation in the same sense as in the Mosses and Liverworts and in the higher plants. In the Bryophytes the sexual reproduction has reached a higher stage of development and a much greater constancy as regards the nature of the reproductive organs. On the germination of the spore there is usually formed a fairly distinct structure known as the _protonema_, from which the Moss or Liverwort developes as a bud[449]. ╭ MARCHANTIALES. I. HEPATICAE. ┤ ANTHOCEROTALES. ╰ JUNGERMANNIALES. The vegetative plant-body possesses a different organisation on the ventral and dorsal side; it has the form of a thalloid creeping plant (Thalloid Liverworts), or of a delicate stem with thin appendages or leaves without a midrib (Foliose Liverworts). In most cases the body of the plant is made up of parenchymatous tissue, showing but little internal differentiation; in one or two genera a few strengthening or mechanical fibres occur among the thinner walled ground-tissue. On the germination of the spore, a feebly developed protonema is produced, which gives rise to the Liverwort plant. Reproduction as in the group Bryophyta. [Sidenote: DETERMINATION OF LIVERWORTS.] The Liverworts have a very wide geographical distribution, and are specially abundant in moist shady situations; they grow on stones or damp soil, and occur as epiphytes on other plants. _Marchantia_, _Pellia_, and _Jungermannia_ are among the better known British representatives of the class.