2.1 From fossil evidence there was, about 400 million years ago, an abundance of all kinds of algae in the seas, lakes, and rivers.
Out of this multiplicity of genera, land plants began to evolve from an ancestral line which had a rather peculiar life-cycle, some elements of which are still discernible in the lives of the highest evolved land plants at the present day, the angiosperms.
This life-cycle involved two consecutive, alternating generations (as they are called) that are very different from one another in structure and in reproductive activity.
One of the generations is usually very small, perhaps only one centimetre or so in length and breadth if we take the prothallus of a fern as our example. In spite of the fact that ferns are quite conspicuous plants it is quite likely that few persons other than professional botanists have ever seen a prothallus. It is a small, flat plate only a few cells in thickness which has no protection against desiccation, only surviving in moist, shaded conditions. It requires some sunlight in order to carry out photosynthesis which will provide food for itself and to start the next generation. On its underside, where there is often a film of moisture next to the soil, spermatozoids (as they are called in plants) are released from the structures producing them (the antheridia) towards the ova. The movement is carried out in almost the same way that sperms move in animal reproduction. The ova are produced singly and retained in special tubular structures called archegonia. These open when the ova are ripe, letting in spermatozoids which have been attracted by chemical substances released in the opening process.
Since all of the cells of the prothallus are haploid, the fusion of the spermatozoid and the ovum produces a diploid fertilized egg. The latter is sheltered in its early development by the prothallus from which it takes the necessary food for growth. It becomes much larger than the prothallus and, once firmly established, the latter shrivels away leaving no trace. The new plant has all of the characters, described previously, associated with successful life on land and where water supply is dependent upon the vagaries of the weather. During dry periods the plant, protected against desiccation, sustains itself upon its water reserves and those of the soil. Once large enough, it develops sporangia within which spores are formed involving a process of nuclear division (meiosis) that produces again the haploid number of chromosomes in each cell nucleus. The resulting spores are scattered by air currents and if they land in a moist, shady situation will germinate to form prothalli. Thus the whole cycle starts again.
It is therefore tempting to make analogies between this life-cycle which involves a stage in which environmental water is vital for fertilization and that of the Amphibia such as frogs and toads where the primarily land-dwelling adults return to water so that sperms and eggs may be set free to bring about fertilization. Pursuing the comparison with the animal kingdom, it is interesting to see that in the same way that land animals eventually freed themselves from the necessity of returning to water to produce fertilized eggs, land plants also modified their basic two-generation life-cycle so that the need for environmental water was avoided.