Botany: The Science of Plant Life

by Norman Taylor

(A1) archegonia, (A2) antheridia, and (A3) the rhizoids. B: Prothallus, showing the young plant with its first leaf (B1), its own roots (B3) and the rhizoids of the prothallus (B2). Drawing and legend for it slightly altered from Kraemer.

in the truest sense, merely a preparation for the process that will produce another fern, it is always known as a prothallus. The prothallus is thus the first stage in the reproduction of ferns, a very simple stage, with only the faintest indication that the thallus might be considered the vegetative and its rhizoids perhaps the rootlike counterparts of foliage and roots of mature ferns. As we shall see presently, even this differentiation has not the significance that such a structure in flowering plants would indicate. There is not, as yet, the faintest indication of sexes that need to mate in order to produce their young. The spore has so far only produced a tiny flat body of green tissues with a few rootlike threads, so unlike the fern from which it started that its true significance, or even the fact that it had ought to do with ferns was not known until about the middle of the last century.

This green cushiony prothallus keeps on growing, its heart-shaped mass becoming divided into an obviously left and right hand side and the rhizoids multiplying in number. They are always borne on the lower side next the ground, or next whatever the prothallus may be growing on. Near the notch of the heart-shaped prothallus are developed a few flask-shaped bodies which contain within them an egg cell or single ovum, the female reproductive body. By a series of changes this egg cell becomes embedded in a mucilaginous material. This flask-shaped body with the female egg cell inside is known as the archegonium. From among the rhizoids there may, at about the same time, be found developing small globular organs that have in them a number of tiny cells, each of which has attached many minute threadlike tails. The globular organs, with their minute, tailed cells are known as antheridia, and comprise the male reproductive equipment. Just as in flowering plants, neither the archegonia (female) nor the antheridia (male) can produce offspring without mating and the method by which this marriage is accomplished differs tremendously both in practice and in its significations from that in phanerogams. In the first place, the male and female reproductive cells are separated by a considerable distance, they are both inclosed in structurally different casings, and the whole operation is so microscopic that insects can be of no service. Nor can the wind do for them what we have seen that it does for the pollen of pines and grasses.

Of the aids to fertilization there remains then only the water, which plays such an important part in the mating of the eelgrass and ditch grass among flowering plants. But in these ferns a very different drama is about to be enacted. The male cells, as we have seen, are provided with slender tails, which are movable. They move, in fact, to such good purpose that the male cell can actually swim in the water. Of course its minute size demands only the merest drop of water, in which it will take the only excursion of its brief life. For just as soon as it is mature, a heavy dew or the tiniest particle of water will set free the little male messengers. The water too has not been without effect on the female cell. More remarkable still, this mucilaginous matter contains in it a substance that acts as a lure to the swimming male cells. In any event they do swim directly to the entrance of the female cell’s abode, through it and to her, when the union is effected. At once there is thrown across the entrance a membrane that excludes all other males, and the fertilization is complete. From this union of the male and female cells a true young fern begins to develop. First a young leaf and roots, finally a stem and in the end, of course, a full-grown fern producing spores, ready to renew the whole process.

Some ferns do not follow all the steps exactly as we have outlined, for all of them have not the structure of the typical one whose life history has been sketched above. In the adder’s-tongue fern, for instance there is a stalklike prolongation from the base of the only leaf the plant bears, on which all the spores are borne. In certain others, as in the ostrich fern, the spores are borne on leaflike growths that serve only this function. Most ferns, however, bear spores on otherwise unmodified foliage leaves and the great bulk of them on the under side of such leaves.

There are several things about the life history of a fern that differ fundamentally from any flowering plant and perhaps the chief is what is known as the alternation of generations. A spore, for instance, can never produce a fern as a seed will always produce a flowering plant. In this respect they are like many insects that always have two or sometimes three different stages in their life history. Only by the complicated method of first a spore then the prothallus, from which archegonia and antheridia are produced, followed by the free swimming male cells fertilizing the female, can a fern reproduce itself. As we shall see in the chapter on the History of the Plant Kingdom, this alternation of generations, the absolute necessity of water in which to carry on the fertilization, and above all the ability of the male cells for free swimming in the water, are all landmarks in the development of plant life. In its simplest form fertilization in flowerless plants is characterized by one or all these processes, as it is in the ferns, while in the flowering plants, the act is accomplished by processes, discussed previously, which, in the development of the plant kingdom, mark a period only comparable, in the history of man, to such tremendous achievements as the acquirement of speech or the ability to make a fire.