In this article we will discuss about the asexual and sexual modes of reproduction in marchantia with the help of diagrams.
Asexual Reproduction in Marchantia:
Asexual propagules – gemmae – occur in a gemma-cup (Fig. 2.6A). On the floor of the cup are seen numerous mucilage papillae and many biconvex discoid gemmae (Fig. 2.7E). A gemma (Fig. 2.7F) is borne on one-celled stalk and is a multicellular discoid body which is thick in the median region and thins out towards the margin.
Each gemma has two or more lateral notches, which are in fact growing points. The gemmae are chlorophyllous structures. However, here and there, there are colourless cells, which give rise to rhizoids, and some oil-rich cells. The gemmae get detached from the cup, due to expansion of associated mucilage cells on absorption of water, and finally the splashing rain drops help in their dispersal.
On a moist substratum the colourless cells of a gemma grow out to form rhizoids and start the absorptive function. Following it, notch meristems regenerate to form new thalli; one from each growing point. On decay of central part of gemma, thalli become independent.
Sexual Reproduction in Marchantia:
The sexual maturity of thallus is revealed by the presence of upright reproductive branches; the antheridiophores and archegoniophores. Each of these branches is of a particular morphology, borne on different thalli towards their apices. These branches originate as minute button-shaped, superficial structures
This genus is dioecious; the sex organs – archegonia and antheridia – are borne on specialized branches, archegoniophores (carpocephala) and antheridiophores, on different thalli (Fig. 2.6B,C). The antheridiophore (Fig. 2.6B; 2.8A) and an archegoniophore (Fig. 2.6C; 2.8B) are modified prostrate branches. This is evidenced by their dorsivetrality.
The morphological ventral side has one or two longitudinal furrows having scales and rhizoids and morphological dorsal side shows an internal differentiation of air chambers (Fig. 2.8B). These specialized branches are an extension of thallus; in their formation is utilized the growing point of thallus, resulting in cessation of further growth.
In a young archegoniophore there is precocious dichotomy: It is followed by a second dichotomy. Yet another dichotomy in each of four lobes results in a rosette-like 8-lobed disc (Fig. 2.6C). After cessation of dichotomy archegonia initiate in acropetal succession on dorsal side of every lobe.
The ontogeny of an archegonium is similar to that in Riccia.
Archegonia mature and fertilization takes place when the disc of archegoniophore is slightly above the thallus surface. After fertilization, the stalk of the archegoniophore starts elongating and there is tissue proliferation in the central area of the disc. This upward growth results in inversion of marginal portion of the disc.
As a result, archegonial necks are directed downwards (Fig. 2.8B) and the position of archegonia becomes reversed – the oldest archegonia are at the periphery of disc and youngest near the stalk. Along with these changes, the central part of the disc, due to intercalary growth, produces long sterile lobes of tissue (rays) which alternate with the archegonia-bearing fertile lobes.
In M. polymorpha, these rays are conspicuous finger-like projections (Fig. 2.9B). After the inversion of archegonia there is formation of one layered plate of tissue on either side of every group of archegonia. This two-lipped pendent-fringed sheath is known as perichaetium or involucre (Fig. 2.8B).
The antheridiophore is similar to archegoniophore in its relation to thallus. It consists of a stalk and a lobed-disc (Figs. 2.6B, 2.8A). The lobed-disc is a result of repeated dichotomies. Each lobe is a flat structure (Fig. 2.8A), bearing antheridia in acropetal succession; older ones towards the centre of the disc and younger ones towards the apex of lobes.
Ontogeny of an antheridium is essentially similar to that in Riccia.
An antheridium (Fig. 2.8A) consists of a short stalk and a globular body, its jacket is a single layer of isodiametric cells enclosing a large number of androcytes. When water falls on the slight concave disc of antheridiophore it is drawn into the antheridial cavities, each enclosing an antheridium. At the time of dehiscence, the jacket cells of antheridium, except the terminal ones, extend in transverse direction exerting a pressure on androcytes.
The terminal cells of jacket disintegrate in the presence of water and the entire mass of androcytes is ejected in a long smoke-like column (Fig. 2.8C). Rain drops falling on the disc of antheridiophore can splash the androcytes up to two feet. Water falling on an archegoniophore even after its upper surface becomes convex, resulting in inversion of archegonia, flows over the edge of archegonia and brings about fertilization.
Also on record are androgynous receptacles. On these the archegonia are borne on the undersurface and antheridia in cavities opening to outside. The occurrence of androgynous receptacies is an indication of a tendency towards monoecism. In M. palmata, an Indian species, female receptacles always end up as androgynous receptacles, particularly when fertilization fails.
As a stimulus of fertilization the stalk of an archegonium elongates, becoming a couple of mm in length. The cells of venter divide to form a 2 to 3-layered calyptra. Also, a ring of cells at the base of venter divide and re-divide to form one-cell-thick collar around archegonia, called perigynium (pseudoperianth).
It ultimately grows around archegonia. Thus, a developing sporophyte has three concentric sheaths (Fig. 2.8B, and Fig. 2.9A), of gametophytic origin; the calyptra, perigynium (pseudoperianth) and perichaetium (involucre). The main function of these sheaths is to provide protection to a developing sporophyte against drought.
The first division of zygote is transverse or so (Fig. 2.9G, H).
Further divisions result in two types of embryos:
(a) Quadrant; a more prevalent type seen in M. polymorpha and
(b) Filamentous, a rare type of embryo seen in M. chenopoda. Cells in a filamentous embryo cannot be related to different parts of sporophyte but in a quadrant-type of embryo the hypobasal half contributes to foot and seta and epibasal half forms the capsule.
The quadrant- embryo is followed by an octant-embryo and further divisions, not in a regular manner, result into a globular stage (Fig. 2.9I). When the embryo has about a dozen or more cells, in circumference periclinal division in the capsular portion results in differentiation of amphithecium and endothecium.
Early in the development of sporophyte, cell divisions almost exclusively in one plane result in a seta (Fig. 2.10A) composed of vertical rows of cells. Late in development of sporophyte, cells of foot region divide to form a massive and bulbous foot; it is meant for anchorage of sporophyte and absorption of food from adjoining gametophytic cells. In Marchantia sporophyte, the cells of foot, seta and capsule are chlorophyllous; indicating that the sporophyte is not fully dependent on the gametophyte.
In the capsule region of sporophyte, the amphithecium remains one-cell thick and forms the wall of capsule. The cells of endothecium divide repeatedly to form a mass of sporogenous cells.
These cells gradually separate from each other and elongate (Fig. 2.10B) along the axis of capsule, and then differentiate into two types:
(a) Fertile cells, broad with dense contents and prominent nucleus and
(b) Sterile cells, which are long and narrow with sparse contents. These two types of cells are almost equal in number.
The fertile cells undergo repeated transverse divisions to form a row of cells (Fig. 2.10B), which remain enclosed in the wall of the parent cell. Each cell in a row is spore-mother-cell. An original fertile cell contributes to 8 or more spore-mother-cells in M. palmata and 32 in M. polymorpha. Later, these spore-mother-cells are set free and each one divides meiotically to form spore tetrads (Fig. 2.10C). The individual spores separate at the maturity of capsule.
The sterile cells elongate further to form long tapering cells. The cytoplasm of these cells gradually disappears and in these cells develops 2 to 3 bands of spirally coiled thickenings (Fig. 2.10D). These are elaters, which, due to their hygroscopic nature, aid in the dispersal of spores.
Spore output in Marchantia has been estimated to be about 30,000 spores per capsule. Since spore-mother-cells are several generations further than the elaters, in M. polymorpha there are 32 spore-mother-cells and 128 spores in relation to just one elater.
During maturation of sporophyte the seta elongates (Fig. 2.9E) and pushes the capsule through calyptra, perigynium and perichaetium. The exposed capsule splits into 6-8 valves (Fig. 2.9F), releasing spores. After dehiscence of capsule, hygroscopic changes in the elaters assist in the dispersal of spores.
The spores are very small, 3 to 12 microns, may be apolar, without a triradiate mark or cryptopolar, not globose but faintly tetrahedral, and readily germinate to produce a gametophyte (Fig. 2.10E). Before germination, a spore undergoes asymmetric division, the smaller cell which is relatively poor in cell content and is achlorophyllous extends to form a germ-rhizoid.
The bigger, chlorophyllous cell forms the germ-filament or protonema. Cell contents of the protonema migrate towards the apex. The apex is cut off from the rest of sporeling by a division. This apical cell divides to produce a quadrant of cell. This stage is followed by plate formation. The expansion of this plate into a thallus is through activity of marginal cell (Inoue, 1960), a characteristic of Machantia.
