Fig. 8
In 1848 the development of the fern was first satisfactorily explained. It was then shown that these plants pass through what has been called, not altogether happily the modern botanist thinks, an "alternation of generations." One "generation," the "sexual," consists of a tiny, green, plate-like object, termed the prothallium ([Fig. 8]). This is connected with the soil by hair-like roots. On its lower surface are borne usually both the reproductive organs of the fern, the antheridia, corresponding to the stamens or fertilizing organs of the flower, and the archegonia, performing the office of the flower's pistils, inasmuch as their germ-cells receive the fertilizing substance produced by the antheridia. But no seeds are formed as the result of this fertilization. Instead of this seed-formation which we note in the flowering plant, the germ-cell in the fern develops into a fern-plant, which forms the "asexual" generation.
Fig. 9, Fig. 10, Fig. 11
First fronds of Maidenhair
The first fronds of this little plant are very small and simple, quite unlike the later ones. For a time the plant is nourished by the prothallium, but as soon as it is sufficiently developed and vigorous enough to shift for itself, the prothallium dies away, and the fern maintains an independent existence. Eventually it produces fronds which bear on their lower surfaces the sporangia containing the minute spores from which spring the prothallia.
For our present purpose it is enough to say that spores differ from seeds in that they are not the immediate result of the interaction of reproductive organs. They resemble seeds in that they are expelled from the parent-plant on attaining maturity, and germinate on contact with the moist earth.
Thus it is seen that the life-cycle of a fern consists of two stages:
First, the prothallium, bearing the reproductive organs; second, the fern-plant proper, developing the spores which produce the prothallium.
Along the moist, shaded banks of the wood road, or on decaying stumps, keen eyes will discern frequently the tiny green prothallia, although they are somewhat difficult to find except in the green-house where one can see them in abundance either in the boxes used for growing the young plants, or on the moist surfaces of flower-pots, where the spores have fallen accidentally and have germinated.
As the fertilization of the germ-cell in the archegonium cannot take place except under water, perhaps the fact is accounted for that ferns are found chiefly in moist places. This water may be only a sufficient amount of rain or dew to permit the antherozoids or fertilizing cells of the antheridium to swim to the archegonium, which they enter for the purpose of fertilizing the germ-cell.