Riccia - Fertilization, Sporophyte Generation

Fertilization

Water is indispensable for the process of fertilization. The antherozoids reach the mouth of the archegonuim through the medium of water. The mucilaginous mass of the neck region absorbs water and swells and as a result the cover cells get separated. This results in the formation of a passage which facilitates the entry of sperms or antherozoid. The antherozoids enter the mouth of the archegonuim, travel through the neck and reach the vicinity of the egg. One of the antherozoids penetrates egg cell and the fertilization is effected. Ultimately by the union of the nuclei of male and female gametes, a zygote is formed. The zygote contains 2n number of chromosomes i.e., the zygote is diploid.

Sporophyte Generation

The diploid zygote is the first cell of the sporophyte generation. This cell secretes a wall around it soon after the fertilization and enlarges in size and nearly fills the cavity of the venter. Afterwards it undergoes division and attains two celled stage. The upper cell is known as epibasal cell and lower cell is known as hypobasal cell.

Sporophyte

The two cells of the embryo (epibasal and hypobasal) divide further and give rise to a four celled stage of the embryo which is followed by eight- celled stage.

At a later stage the embryo is differentiated into two regions. The outer layer is amphithecium and the inner mass of cells is endothecium. The amphithecium is protective in nature whereas endothecium gives rise to a mass    of    sporogenous cells.

Sporemother cells are produced from sporogenous cells. Spore mother cells undergo meiotic division and tetrads are produced. Thus a tetrad of four spores is formed. The spores becomes separated from each other only on maturation. The spores are haploid in nature. On the death and decay of the thallus, the spores get free from the sporogonium.

Structure of the spore

The mature spore is three layered. The outer most, cutinized layer is exosporium, the middle layer is mesosporium which is thick walled and consists of three concentric zones. The inner most layer is endosporium. The spore is the beginning of the gametophytic generation.