In this article we will discuss about: 1. Plant Body of Mosses 2. Reproduction of Mosses 3. Life-Cycle of Mosses.
Plant Body of Mosses:
The plant body of mosses is very simple. It has a small branched stem with spirally arranged simple leaves (Fig. 201). Multicellular hairy outgrowths, called rhizoids, develop from the base of the stem which carry on the functions of roots, true roots being altogether absent. Mosses have no vascular tissues; some elongated cells at the centre of stem and midribs of leaves are solely concerned with conduction of water and food.
Reproduction of Mosses:
The plant bears gametangia at the apices of the stem and branches. So they are called gametophores or gamete-bearers. A longitudinal section of the apex reveals that antheridia and archegonia remain intermixed with many sterile hairy processes called paraphyses (Fig. 203).
Mosses may be monoecious or dioecious. The leaves of a male shoot often spread out in the form of petals of a flower and hence commonly called ‘moss flower’, though technically, it has nothing to do with a flower.
Each antheridium is a club-shaped body with a short stalk. It is normally green in colour but turns orange-red with maturity. Antheridium contains many spermatozoid-mother cells in the central region, which produce a fairly large number of sperm cells. Every sperm cell develops into a bi-ciliate spermatozoid or antherozoid (male gamete).
They remain within the antheridium till moisture is available. When the mature antheridium gets moisture, the mucilaginous contents surrounding the sperm cells absorb water and swell up. A pressure is set up, as a result of which the sperm cells are liberated in a mass through an apical opening.
Outer mucilaginous wall of each sperm cell dissolves in water, and a naked bi-ciliate spermatozoid is set free (Fig. 202). It moves about in surrounding film of water.
The female shoot similarly bears archegonia with paraphyses (Fig. 204, left). Each archegonium is a distinct flask-shaped body with a massive swollen lower portion called venter and a long projecting neck. The venter contains two cells, viz. the lower and the larger one is the female gamete called egg or ovum, and the other one is known as ventral canal cell.
The neck contains a few cells called neck canal cells (Fig. 204, right). With maturity, the tip of archegonium opens and all cells excepting the egg disintegrate forming mucilaginous mass. Thus a thorough passage is established to the only surviving cell, egg.
Further, archegonium secretes chemical substance, sucrose, to attract the male gametes. A large number of them rush towards the egg; one of them passes down the canal and fuses with the egg. This fusion is fertilisation, and the zygote formed is the oospore, which soon secretes a wall around itself.
Oospore begins to divide inside the venter of the archegonium and forms the embryo-sporophyte, which by further division and elaboration gives rise to the complex but important structure called sporogonium of moss.
The sporogonium has three parts:
(i) Lower foot, by means of which it remains attached to and draw water from the plant,
(ii) A long stalk, called seta, which conducts water absorbed by foot, and
(iii) The swollen terminal pear-shaped portion called capsule which is destined to produce the asexual spores (Fig. 201).
It goes without saying that capsule is covered by a delicate and temporary cap or hood called calyptra, which is soon blown off by wind. During the formation of sporogonium the seta elongates rapidly, and as a result, the upper portion or archegonium ruptures and is carried upwards by the elongating seta.
Thus the calyptra is nothing but the remnants of the archegonial wall. The capsule is a complicated multicellular body, having arrangements for formation and liberation of spores. A longitudinal section of the capsule would reveal the following structures (Fig. 205).
The basal solid portion is called apophysis. Capsule has an outer epidermis with stomata. Next to epidermis there is a zone of parenchyma cells with chloroplasts called sub-epidermis. Internal to that, there are many air cavities traversed by delicate strands of cells.
Two layers of compact protoplasmic cells are found next towards right and left, surrounding the central sterile parenchymatous portion called columella. The protoplasmic cells constitute the sporogenous tissue or spore-sac. Every cell of spore-sac functions as a spore-mother cell. It undergoes reduction division and gives rise to a tetrad of spores.
A multicellular lid, called operculum, is present at the mouth of the capsule. At the region where operculum meets the main body of the capsule, there is a row of large cells, called annulus, with cuticularised walls. The last layer of operculum is known peristome.
It consists of cells with teeth-like projections which respond to atmospheric changes in humidity. When atmosphere is moist, they fit in tightly with the internal cells; and when air is dry, they withdraw. After the formation of spores in the capsule, annulus breaks which is followed by the withdrawal of peristome teeth.
As a result, the whole operculum is blown off, making a clean passage for the liberation of the spores. Spore, as we know, is a unicellular body with two coats, intine and exine. When conditions are satisfied, spore germinates to form a green thalloid filamentous body called protonema (Fig. 206).
The protonema bears many lateral buds, each of which develops into a new moss plant, thus completing the life-history.
Life-Cycle of Mosses:
Moss plant reproduces by two methods, sexual and asexual. Plant itself bears the sexual units, sperms and egg. This part is called gamete-bearing part of gametophyte. When the two gametes fuse, zygote is formed, which develops into the sporogonium. Sporogonium in its turn bears asexual spores. That part is spore-bearing generation or sporophyte.
The two generations regularly alternate in the life-history. This is the phenomenon of alternation of generations. Two generations differ in form and structure, in methods of reproduction and also in number of chromosomes in the nuclei of the cells.
The gametophyte always contains reduced or haploid number of chromosomes, represented as ‘n’ number; whereas the sporophyte has double or diploid number of chromosomes represented as ‘2n’ number. Two cardinal points in the life-history determine the limits of gametophyte and sporophyte.
They are:
(i) Reduction division in the spore-mother cells with subsequent formation of spores, which marks the beginning of gametophyte, and
(ii) Fertilisation involving union of two gametes which restores the ‘2n’ number (‘n’ being contributed by each gamete), and initiates the sporophyte (Fig. 207).
In moss gametophyte is the more prominent generation represented by the plant itself and the protonema; and sporophyte represented by sporogonium is partially dependent or semi-parasitic on the gametophyte.
Calyptra, the temporary hood on the capsule, the most important part of sporophyte, really belongs to gametophyte. Another point to be noted is that in the same life-history moist condition is necessary for liberation of spermatozoids and dry air for dispersal of spores.