In this article we will discuss about:- 1. Plant Body of Ferns 2. Reproduction of Ferns 3. Life-Cycle.
Plant Body of Ferns:
Most of the ferns are small perennial herbs. Tree ferns with long pillar-like stem and a crown of leaves at the top are found only in tropical countries. The plant body is differentiated into steam, roots and leaves with well-developed vascular bundles. The stem is usually a creeping underground rhizome bearing many adventitious roots. It remains covered by a large number of persistent leaf-bases.
Pinnately compound leaves, often called fronds, spring up from the rhizome. The leaves, when young, remain coiled inwards, thus exhibiting circinate vernation (Fig. 208).
The underground rhizome, young leaves and rachis of adult leaves remain covered by brown hairy scales called ramenta (singular— ramentum). This is a safeguard against drought. Circinate arrangement of young leaves and presence of ramenta are characteristic features of ferns.
Reproduction of Ferns:
Unlike moss, the fern plant is the spore-bearing generation or sporophyte. The leaflets bear some brown kidney-shaped dots, called sori (sing, sorus), on the under surface (Fig. 209). They are usually present directly on the veins or vein-endings.
A section of a leaflet passing through a sorus shows that it (sorus) is a collection of sporangia, which develop from the soft parenchymatous cushion-like outgrowth called placenta.
The sporangia, when young, remain protected by a curved outgrowth of the leaf-tissue, called indusium (Fig. 210). Each sporangium has two parts, viz. a multicellular stalk surmounted by the swollen body called the capsule. The capsule has the outer wall made up of one layer of cells. It encloses a definite number of large cells called spore-mother cells.
The number is usually sixteen; in some cases it may be twelve. Every spore-mother cell undergoes reduction division to produce a tetrad of spores. Reduction division with subsequent formation of spores, marks the beginning of gametophytic generation.
Along the margin of the capsule wall there is a row of cells with inner and radial walls cuticularised. This row covering the major part of the capsule is called annulus. A few thin-walled cells forming the weak spot, so to say, on the capsule wall constitute, what is known as stomium (Fig. 211).
After formation of spores the annulus straightens up in response to atmospheric changes in humidity, and the capsule bursts at the stomium liberating the spores, which are easily carried away by wind.
Spore is a unicellular body with two coats, outer thick exine and thin inner coat, intine. Under suitable conditions spore germinates. Exine bursts and intine comes out in form of tube which, by repeated cell division, gives rise to a green flat heart-shaped thalloid body called pro-thallus.
It develops unicellular hairs, called rhizoids, from the under surface for fixation and absorption of water and mineral matters. Thus the small pro-thallus, barely TO cm in size, is an independent body. Pro-thallus bears the sex organs, and so represents the gametophyte of fern. Antheridia are located at the basal part amongst the rhizoids and archegonia near the apical notch (Fig. 212).
An antheridium is a spherical structure with an outer jacket of sterile cells. It encloses a good number of spermatozoid-mother cells; each of them produces a spermatozoid or antherozoid (male gamete).
In presence of water sperms, surrounded by remnants of the mother cells, are liberated from the antheridium through an apical opening. The spermatozoids are naked spirally twisted multi-ciliate bodies (Fig. 213, left). They swim freely in surrounding film of water.
Archegonium is a flask-shaped body with the basal swollen portion, venter, and the short slightly curved neck. Venter remains embedded in the body of the pro-thallus, only neck projects out. Venter contains the female gamete, egg or ovum, and a ventral canal cell; and the neck contains a bi-nucleate neck canal cell (Fig. 213, right).
At maturity of the archegonium, all the cells excepting the egg dis-organise, an opening is formed at the tip, thus establishing a thorough passage to the egg. Besides mucilage, archegonia secrete chemicals, malic acid and salts of malic acid, which attract the sperms.
Quite a good number of them move towards the archegonium. One passes down the canal and fertilizes the egg. The oospore, thus formed, soon secretes a wall around itself. By repeated cell division oospore gives rise to the multicellular embryo, which develops into a new fern plant, thus completing the life-history.
Life-Cycle of Ferns:
The above life-history shows that it has two distinct parts, the plant itself reproducing asexually by spores and the pro-thallus reproducing sexually by gametes. Sporophyte and gametophyte, as they are called, regularly alternate in the life-history, establishing ‘alternation of generations’.
Sporophyte, the more prominent generation, contains, diploid or ‘2n’ number of chromosomes in the nuclei of the cells. Fertilization is the starting point of this generation and continues up to spore-mother cells.
As soon as there is reduction division of the spore-mother cell, number of chromosomes is reduced to half and gametophyte begins. So spore is the starting point of gametophyte, which continues right up to male and female gametes before their union.
The gametophyte, though much smaller and inconspicuous, is independent of the sporophyte. The two cardinal points, fertilization and reduction division, determine the limits of the two generations (Fig. 214).