The Marchantiales are Hepaticae in which the thallus is composed of several distinct layers of tissue, of which the uppermost, the chlorophyllbearing layer, nearly always encloses air chambers, which have communication with the exterior through pores. The rhizoids are of two kinds, smooth and tuberculate. The sex organs are generally united in receptacles often borne on long-stalked antheridiophores and archegoniophores respectively. The order includes relatively few British genera, though some of the species are among the commonest Liverworts.
Consider as example of this order Marchantia polymorpha.
Marchantia polymorpha, is more complicated than Pellia both in the structure of the thallus and also in the sexual reproductive organs. The general morphology of the thallus of the two plants is much the same. It is flat, dichotomously branched, possesses a central midrib and develops rhizoids on the lower surface in thesame way as in Pellia. There is, however, this considerable difference, that there is no special apical cell, as in Pellia, but a group of meristematic cells at the apex.
When we come to examine the internal anatomy of this thallus we find that there is 'considerably greater differentiation of tissues. In Pellia the thallus was composed of a more or less uniform parenchyma; in Marchantia we can distinguish a number of distinct layers. Starting from the upper surface there is first of all a single layer of epidermal cells. Embedded in this are special ventilating pores which allow the passage of gases through the epidermis. These pores are surrounded by four rows of four cells, the rows forming rings one above the other, leaving an aperture in the centre.
Thus the pore wall lies half above and half below the epidermis. Below the epidermis there are a number of chambers, within which are short, sometimes branched, filaments of cells, which develop from the flat base of the chamber, which is composed of cells similar in shape to those of the epidermis. The chambers, as seen from above, are diamond-shaped areas and are bounded by walls of three or four cells in height, which unite the epidermis above with the parenchyma below. All these cells, and particularly the filaments within the chambers, are dark green in colour, due to the presence of numerous ovoid chloroplasts. Each chamber lies directly under one of the ventilating pores, and there is no doubt that the green filaments are the principal centres of photosynthesis in the thallus.
The lower layers of the thallus are made up of relatively large parenchyma cells similar to those in the thallus of Pellia, among which are a number of large mucilage cells. The lower surface is covered with an epidermis composed of cells similar to those of the upper epidermis, and from these cells two types of outgrowths arise. Firstly, there are the rhizoids, which are unicellular and contain very little protoplasm, and, secondly, a number of multicellular scales or amphigastria, which form two overlapping rows, one on each side of the midrib. The amphigastria no doubt serve to retain moisture below the thallus which the rhizoids can absorb. These rhizoids, like those in other members of the Marchantiales, are of two kinds; one set are smooth-walled, the others show a large number of minute thickenings or tubercles \yhich project from the walls into the lumen of the rhizoid cells. The smooth rhizoids emerge from behind the amphigastria and penetrate the soil. The tuberculate rhizoid's lie parallel to the underside of the thallus and form a system of conducting strands leading forwards to the growing point, where water is readily absorbed. It may be correlated with this that Marchantia can grow in much drier places than Pellia can. Damaged rhizoids may be regenerated, secondary and even tertiary rhizoids proliferating within the primary one.
There are t\yo methods of vegetative reproduction in Marchantia. The first depends upon the ageing of the vegetative cells. It appears that the cells of the thallus only live for a month or two, and as new cells develop at the apices of the branches those further back die and eventually disorganize. In this way the indiyidual branches become separated, and each continues life independently.
The second method of yegetative reproduction is by means of gemmae.
These gemmae are lens-shaped masses of cells, which when detached develop into new plants. They are developed in the base of small cup-shaped organs termed cupules, which are produced along the midrib. The gemmae arise from a superficial cell which divides into two. The upper cell enlarges and divides further to produce a group of cells with an indentation at each end which marks the presence of a growing point. 'When mature the gemma is liberated by the disorganization of the stalk formed by the basal cell. On its liberation the gemma develops into a fresh thallus, growing from both ends simultaneously. Gemma formation usually takes place during the autumn and spring, and affords a very rapid and successful means of dispersal during the growing season.
Marchantia polymorpha is dioecious, that is to say, that the male and female reproductive organs are borne on different plants. The sex organs are generally produced during the summer, and differ markedly from those of Pellia. Instead of being immersed within the tissues of the thallus, the sex organs are elevated upon special branch structures, termed respectively the antheridiophore, which bears the male organs, and the archegoniophore, which bears the female organs. Both structures are developed at the growing apex and form a direct continuation of the midrib, but they grow vertically upwards- through the apical indentation in the thallus until they are as much as 3 cm. in height. In general structure these organs resemble the thallus, and we must regard them as branches of the thallus, borne on the end of stalks, rather than as purely reproductive bodies.
The antheridiophore consists of a stalk bearing at its apex a disc-shaped cap. In transverse section this stalk is seen to consist of a narrow thallus branch, the margins of "'hich are curled downwards and inwards until they almost meet below the midrib, thus forming two vertical channels running up the stalk.
The disc is made up 'of eight separate lobes, the indentations between which represent the apices of a corresponding number of branches. disc has all the features of the anatomical structure of the thallus.
This There is the same type of epidermis with its ventilating pores and the same large chambers containing assimilatory filaments; amphigastria are present, and rhizoids are given off from the lower surface and pass dovynwards through the channels of the stalk. The antheridia are embedded in cavities sunk in the tissues between the chambers and lined by a .layer of cells whose walls are often deep purple in colour. Each cavity is oval in shape, has a tubular opening upwards, and contains a single large antheridium, which is attached by a short stalk at the base. The development of the antheridium is essentially similar to that in Pell£a, and the antherozoids when mature are liberated by. the bursting of the antheridium and are extruded out of the chamber in a drop of milky liquid. They are subsequently transferred to the archegoniophore, possibly through the agency of dewar rain falling on the upper surface of the antheridiophore. '\Iites have been found in the chambers of the archegoniophore, and since sucrose is secreted by both antheridia and archegonia it has been suggested that a form of " insect pollination" may occur in Marchantia.
The archegoniophore is similar in general structure to the antheridiophore. It is made up of a stalk, ,which is generally longer than that of the antheridiophore, and a terminal disc, which is also branch-like. The growing apices of these branches, however, turn downwards and inwards towards the stalk so that their positions are marked by deep indentations in the disc. The tissue between these growing points projects as long finger-shaped processes, like the ribs of an umbrella. The archegonia are formed in small triangular patches near each of the down-turned apices. Each patch is surrounded by an involucral membrane, which is developed from the edges of the ribs and hangs down vertically to protect the developing archegonia. Some twelve to sixteen archegonia are produced in each of these enclosures, and develop in succession from the outside imvards. Their development is more or less similar to that in Pellia, and it is unnecessary for us to consider it in detail. There is, however, one difference, that each archegonium is surrounded by an individual collar of cells called the perianth.
The stalk of the archegoniophore is provided with the same two longitudinal grooves tbat we have noted were present in the antheridiophore, and it has been suggested that the function of these grooves is to provide a water channel in which the antherozoids may swim down the antheridiophore and up the archegoniophore to reach the archegonia, We do not really know precisely how the male gametes find their way to the archegonia, but from the fact that fertilization of the oosphere is effected we know that they must do so. When we consider not only the distance between the sex organs and the ground but also the fact that the sexes are on different plants, we can clearly see the difficulties in the way of fertilization by a microscopic antherozoid, and the advantages of having a good method of vegetative propagation.
The result of fertilization is the development of a sporogonium in a way similar to that in Pellia .• The oospore divides into eight cells, four. of which form the foot and the seta, whilst the upper- for cells give rise to the capsule. The details of this development are sufficiently similar to those in Pellia for it to be unnecessary for us to consider it in detail. It is interesting to note, however, that during the development "of the sporogonium the stalk of the archegoniophore may elongate considerably, but not the stalk of the sporogonium, as in Pellia, whence we are forcecl):o conclude that the reaction produced as a result of fertilization is not restricted solely to the embryo, but extends also to the tissues surrounding it. As the sporogonium hangs downwards the elongation of its stalk would merely carry the spore capsule down to the ground.
When mature the sporogonium becomes yellow in colour and splits open by from four to six valves. The spores with their accompanying elaters are shed, and, due to the elevation of the sporogonium on the archegoniophore, may possibly be better distributed than is the case in Pellia. The elaters function like the annulus of a Fern sporangium by means of an internal \yater tension set up as the elaters dry. This causes t\yisting, which is violently released when the water tension breaks down, causing the elater to spring back explosively into its normal form, thus scattering the spores. On reaching the ground the spores germinate immediately to produce a tiny filament, from which a fresh Marchantia thallus is produced. Since the sex organs are developed on different plants it follows that the spores must be of two kinds. As we shall see later in our consideration of the Laws of Inheritance it is probable that male and female producing spores arise in equal numbers, but female plants are commoner than males.
ALTERNATION OF GENERATIONS
The life history of Marchantia, it will be seen, is similar to that of Pellia. Apart from the purely asexual production of gemmae there is a definite alternation between a gametophyte, the thallus on which the gametes are borne, and a sporophyte in which the spores are produced. In so far as the sporophyte develops within the archegonium and derives its nourishment therefrom there is little difference in the two examples. In fact a relatively small and parasitic sporophyte is characteristic of the whole of the Bryophyta, and it is not until we pass to the Pteridophyta that 'vve shall find any change in its importance.