Biology of Ferns, Liverworts and Mosses

These are the most primitive of land plants. Most of the liverworts have thalli like those of the wracks, but lying flat on the ground, to which they are anchored by unicellular hairs called rhizoids which are similar to root hairs of higher plants. The mosses have stems bearing leaves and multicellular rhizoids, but have no true roots. Both types of plants are called gatnetophytes and produce sperm and eggs. The sperm are produced in antheridia while the eggs are retained in flask-shaped structures called archcgonia, to which the sperm swim when the plants are covered with dew. The fertilized egg develops into a different kind of plant called a sporophyte, which is parasitic on the gametophyte and which produces spores that are dispersed by the wind. On germination these spores give rise to new gametophyte plants. The liverworts and mosses, therefore, display alternation of generations, the gametophyte generation alternating with the sporophyte generation in the life-cycle.

The Male Fern

The male fern is found in damp and shady woods, and is often grown in gardens. The large leaves or fronds grow to a height of 2 or 3 ft. and spring from a short, thick, horizontal, underground stem, termed a rhizome. The rhizome is covered with leaf bases which remain as stumps after the upper parts of the leaves have died down. The rhizome grows obliquely upwards, and its growing point is covered by the young leaves, which are rolled inwards from the tip and gradually unfold as they grow. Each frond has a long central stalk from which lateral branches are given off on each side. These lateral branches are termed pinna?, and are also divided into small segments which are pointed and slightly toothed. The young leaves and also the main stalk of the fully grown fronds are covered with small brown scales termed ramenta. Slender brown adventitious roots spring from the bases of the leaves.

Reproduction

The mature leaves become sporophylls, I.e. leaves producing spores. On the under sides of the pinnae of the fronds appear kidney-shaped patches overlying the side veins. Each is known as a sorns and consists of a stalked umbrella-shaped structure called the indusium beneath which develop a number of sporangia arising from the swollen base of the stalk of the indusium , A. Each sporangium is a stalked body with a flattened oval head. Running round two-thirds of the edge of the head is the annulus whose cells have much thicker inner walls, all the outer walls being thin. When the spores within the sporangia are ripe, during dry weather, the indusium dries and shrivels, while the sporangia turn brown. As the sporangia become exposed, their cells lose water and a state of tension is set up in each annulus, the thin outer cell walls of which become drawn in by the remaining water. As a result the annulus straightens out, splitting the wall of the sporangium transversely across the middle region and exposing the spores for dispersal by the wind , C. When completely dry, the thin outer walls of the annulus snap and the annulus flies back into its original position, jerking out any remaining spores , D; since the spores are very light they are widely dispersed, and those which alight in damp places germinate in warm weather. Each produces a flat, heart-shaped structure about in. long, which is known as a prothallus. Springing from the under side of the prothallus are many hair-like processes termed rhizoids, which attach it to the ground and act as roots. The cells of the prothallus are thin-walled and contain chloroplasts, so that the plant is able to make its own food.

The sexual organs are borne on the under side, where they lie in the surface film of moisture present between the plant and the soil. Archegonia develop near the apex, whilst the antheridia lie among the rhizoids near the base. The archegonia are simple flask-shaped structures which ripen sucoessively, each one containing a single large female gamete, the ovum, lying in the body of the flask. The antheridia are spherical, and when ripe shed their gametes or antherozoids into the surface film. Each is a spiral with a tuft of short flagella at one end, and swims towards the archegonia, being attracted by a secretion of malic acid. One passes down the neck of a ripe archegonium and fuses with the ovum, forming a zygote. This develops a foot which bores its way into the prothallus, feeding parasitically upon it. A primary root grows down into the soil, a stem grows horizontally, and a simple frond grows upwards to the light to make food. For some time the young plant is dependent on the prothallus for its nourishment, but as the roots and leaves of the fern plant develop, it gradually becomes independent and the prothallus withers and dies.

Alternation of Generations

In the life history of the fern, therefore, there is a regular alternation of the two different structures, namely, the fern plant and the prothallus. Since the fern plant produces the spores it is termed the sporophyte generation, while the prothallus, which gives rise to the gametes, is termed the gametophyte generation. The spore develops into the prothallus, which forms the gametes. From the union of male and female gametes a zygote is produced, which grows into the fern plant. As in the Bryophyta, this regular alternation of sporophyte and gametophyte is termed the alternation of generations. The fern sporophyte is a much larger and more complicated structure than the gametophyte. It is adapted for life on land and requires dry conditions for the dispersal of the spores. The prothallus, on the other hand, is a small simple plant which requires a moist environment in order that the essential process of fertilization may take place.

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