In this article we will discuss about the process of reproduction with the help of diagrams.
Funaria is monoecious and protandrous. A young plant to begin with produces antheridia at its apex (Fig. 4.1 A) and on a branch appear archegonia. Towards maturity, the branch bearing a developing sporophyte (Fig. 4.1G) tends to dominate and its main stem becomes inconspicuous, appearing as an antheridial branch.
The leaves around antheridia are large and conspicuously form a rosette (Fig. 4.1 A). These are known as perigonial leaves. The apex of the main shoot is convex, it appears reddish brown against transmitted light and bears numerous antheridia, in various stages of development (Fig. 4.2A), mixed with uni-stratose sterile structures; the paraphyses.
The paraphyses are ‘filaments’ of 4—5 cells. Each filament is narrow at the base and its cells broaden towards distal region. The apical cell of a paraphysis is invariably globose. The enlarged apical cells of different paraphyses meet over the developing antheridia and provide them protection and moisture. The cells of paraphyses are ascribed to retain moisture and prevent undue drying of antheridia.
A mature antheridium has a short massive stalk and club-shaped body. The jacket is single-layer of polyhedral cells containing numerous chloroplasts which turn brown at maturity. The original apical cell of developing antheridium falls in line with wall cells but remains distinct from others by being larger than the rest and has been described to function as an “operculum” which detaches at the time of dehiscence of antheridium.
Dehiscence of an antheridium is possible when water in the form of rain drops falls on it. The apical cell of an antheridium ruptures and then disorganizes releasing a mass of androcytes. The androcytes spread as a film over air-water interface, metamorphose into biflagellate spermatozoids and are splashed by falling rain drops to the archegonia. Rosette-like perigonium serves as an effective splash-cup from which rain drops can disperse the antherozoids to some distance—from male to female gametophyte.
An archegonium is a flask-shaped structure (Fig. 4.2B) with a long narrow oblique neck of many neck canal cells, a dilated venter with a venter canal cell and an egg. The single-layered jacket of archegonium is of six rows of cells; at the venter it may become two-layered.
In a mature archegonium, the neck canal cells and venter canal cell disintegrate forming a mucilaginous mass and terminal cells of neck gap apart leaving a mucilaginous passage for sperms to reach the egg. Entrance of antherozoids to an archegonium is a chemical-mediated response. Union of male and female nuclei is complete within 10 hours. After fertilization, a zygote secretes a wall, enlarges and divides to give rise to an embryo.
The sporophyte (Fig. 4.1B) is differentiated into three parts—foot, seta and capsule. The foot is a dagger-like conical structure embedded in the tissues of gametophyte, and serves for anchorage and absorption of nutrients. Seta is long, slender, wiry structure bearing the capsule.
The capsule in turn is differentiated (Fig. 4.3A) into three regions:
(a) Apophysis, the sterile basal portion of the capsule, it is in continuity with seta,
(b) Theca or body of capsule, which is the fertile region, and
(c) Operculum, the apical region of the capsule.
Apophysis shows tissue differentiation (Fig. 4.3B). In the centre is a conducting strand which is continuous with the conducting strand of seta. Surrounding the central tissue is a zone of richly chlorophyllous, transversely elongated cells with conspicuous intercellular spaces. The epidermis of apophysis has stomata. Apophysis is the main photosynthetic region of the capsule.
Body or theca of the capsule is a highly differentiated (Fig. 4.3A) region and can serve as a good example of cellular differentiation in relation to its function. The outermost 2-3 layers of cells form the wall of the capsule. Inside a distinct epidermis, there are two layers of compact parenchyma which become one-layered in the lower region and merge with the apophysis.
The cells contiguous with the inner wall layer are richly chlorophyllous, the innermost layer of cells has large air spaces (trabeculae) bound by filamentous partition of chlorophyllous cells. Towards the region of apophysis the air spaces are more conspicuous. Following this zone is a region of narrow non-chlorophyllous cells—the outer cover of spore sac. The spore sac is a zone of sporogenous cells.
It is bound internally by a layer of cells, inner cover of spore sac. The central part of theca is of thin-walled parenchymatous cells, the columella. The basal attenuated end of columella extends into the region of apophysis below and the conical apical portion of columella merges into the region of operculum above.
The operculum, the conical apical region of capsule, is another highly differentiated region serving as an example of cellular differentiation as well as organogenesis in relation to its function. The operculum is marked off from the body of capsule by a constriction. Just below the constriction is a circular diaphragm of 2 to 3 layers of radially elongated cells, forming the rim of the capsule.
Immediately above the rim is annulus (Fig. 4.3A), five to six super-imposed layers of epidermal cells. The two lowest layers of annulus cells are elongated and thin-walled, other cells are thick-walled. Attached below the edge of the rim is the region of peristome – two rings of 16 peristome teeth.
On the outer ring 16 peristome teeth are very conspicuous by their red colour and thick transverse thickening bands; the 16 teeth of the inner ring (Fig. 4.4A) are delicate and colourless, their bases are covered by outer peristome.
During final maturity of the capsule there is loss of water. Dehiscence of capsule is due to dehydration and consequent stress on different tissues. The delicate cells of annulus act as a weak point. The operculum is separated from the body of capsule exposing the peristome teeth (Fig. 4.4A).
Shedding of operculum is due to drying and shrivelling of:
(a) Thin-walled opercular cells below the epidermis,
(b) Thin-walled cells at the base of annulus, and
(c) Upward hygroscopic movement of peristome teeth. The peristome covers the spore sac.
However, on exposure and drying, the outer teeth bend backwards with a jerk exposing the spore sac. Along with these, the twisting and untwisting of seta influenced by humidity and wind bring about dispersal of spores.
A spore readily germinates (Fig. 4.3D) to form one or two germ tubes. These germ tubes elongate to produce a branched filamentous protonema. To begin with, the protonema consists of highly chlorophyllous short cells with transverse septa. This stage of protonema is described as chloronema.
The chloronema matures into caulonema, the shoot-producing protonema. Maturity of chloronema into caulonema is auxin-mediated response. The caulonema comprises erect filaments (similar to chloronema) and prostrate filaments with elongated cells, oblique septa and brown contents.
From the cells of prostrate filaments develop an initial, either for a branch or a bud (shoot) or gametophyte. The branch initial divides transversely to produce a branch but a bud-initial divides by oblique partition walls to differentiate an apical cell with three-cutting faces which give rise to stem and leaves (Fig. 4.3E). On caulonema, the formation of shoot-bud is cytokinin-mediated response. Rhizoids develop from the base of a bud.