Table of Contents
Gametophyte and Sporophyte of Funaria:
Gametophyte of Funaria:
A gametophyte in Funaria is represented by a small leafy plant body and a thread-like structure called protonema. A gametophyte is a short but erect structure that is about 1-3 centimetres in height. It is green in colour and shows radial symmetry. It is differentiated into three parts.
(1) Stem- Stem is not a true stem and is analogous structure to a true stem. It is cylindrical and erect showing radial symmetry. It is green in colour in the upper region and brown at the base. It produces a single branch that arises laterally and is extra-axillary near the base. The main axis acts as the male shoot, while the lateral branch acts as the female shoot.
(2) Rhizoids- They arise from the base of the stem and are brown in colour. The brown colour is due to the colour of the cell wall and not due to any content in the cytoplasm. They are multicellular and are characterized by the presence of oblique septa which increase the efficiency in absorption. The Rhizoids are of different types-
- Main Rhizoids which arise directly from the base of the stem and are thick plant like structure having wide lumen.
- Primary laterals arise from the main Rhizoid as side branches having comparatively narrow lumen.
- Secondary laterals arise from the primary laterals and have very fine lumen.
(3) Leaves- Leaves are green in colour and are analogous structures to the true leaves. These are broad, ovate in their shape having an unbranched single mid-rib. They are sessile and arranged spirally in three rows along the stem. The leaf margins are smooth and the apex is pointed and spiny.
Sporophyte of Funaria (Sporogonium):
A fully grown sporophyte has the following three parts-
(1) Foot- Foot is the basal bulbous part of the sporogonium which grows downwards and come in contact with the conducting strand of the female shoot from which it gets water and mineral salts. The cells of the foot are haustorial in function.
(2) Seta- Seta is the slender elongated structure that raises the capsule high in the air. It also connects the foot with the capsule. It is green when young, but later on, it becomes brown in colour. It consists of the epidermis, cortex and central conducting strand.
(3) Capsule- Capsule is a pear-shaped structure and is green. It can thus manufacture its own organic food. The young sporogonium is contained within the archegonium, but as the seta elongates, the archegonium ruptures and its upper end is carried up by the developing capsule over which it forms a cap-like structure called Calyptra. The capsule of moss sporogonium has a complicated and elaborate structure and consists of three parts-
(I) Apophysis- Apophysis is the basal sterile part of the capsule and is in contact with a seta. It is a solid mass of cells covered over by epidermis having true stomata with two guard cells. Each stoma opens down into the sub-stomatal chamber. Beneath the epidermis is several layers of cells containing chloroplasts. This part of the capsule is photosynthetic in function.
(II) Theca- Theca is the middle fertile part of the capsule and has a rather complicated structure. The central region is occupied by a barrel-shaped column of colourless sterile cells called the Columella. It is narrow below and expanded above and is an upward continuation of the central strand of Apophysis. Around the columella is a narrow spore sac, the outer wall of which consists of two or three layers of cells, while the inner wall is single-layered. Surrounding the spore sac is an air-space traversed by delicate strands of cells called trabeculae. Outside this is three or four-layered walls of the capsule. The cells of the inner layers contain chloroplasts. This is bounded on the outside by a single-layered epidermis.
The spore sac contains several cells called the spore mother cells. Each of these has a diploid number of chromosomes and by two successive divisions give rise to a group of four spores. The first of these is a reduction division so that the spores are haploid.
(III) Operculum and Peristome- The upper region of the capsule consists of the operculum and Peristome. The operculum makes the lid of the capsule and lies on the top of it. The operculum (lid) is 4 or 5 layers in thickness. When the capsule dehisces, the operculum comes away as a circular, cup-shaped lid. At the base of the operculum, there lies a special ring-like layer of epidermal cells around the capsule, called an annulus. It is by the rupture of the annulus that the capsule dehisces. When the operculum falls off, the top of the capsule is seen to be furnished with one or two rows of thickened tooth-like projections constituting the peristome. Their teeth are hygroscopic and help in the gradual dispersal of the spores from the capsule.
Alternations of Generations:
The moss plants are gametophytes (gametophores). These gametophores bear antheridia and archegonia on the separate branches of the same plant. The antherozoids are produced from the antheridium and the egg is produced within the centre of the archegonium. They are haploid (n). Soon after fertilization, an oospore (2n) is produced. Here, the male and female nuclei are fused and thus, the oospore becomes diploid (2n) in nature. The oospore is the first phase of the sporophytic generation. Later on, the sporophyte is produced and it is differentiated into foot, seta and capsule. In the capsule, the spore mother cells are produced. They represent the last phase of the sporophytic generation. The spore mother cells divide by meiosis and tetrads of spores are produced. The spores are haploid and they represent the beginning of the gametophytic generation. This way, there is an alternation of generations. The gametophyte is independent and gives rise to a parasitic sporophyte upon it. The sporophyte produces spores and the spores again give rise to gametophytic plants after germination. Here, the two generations (gametophytic and sporophytic) alternate with each other and this is called the alternation of generations.