How to Know the Ferns (7th ed)/Chapter 4
FERTILIZATION, DEVELOPMENT, AND
FRUCTIFICATION OF FERNS
Until very recently the development of ferns, their methods of fertilization and fructification have been shrouded in mystery. At one period it was believed that "fern-seed," as the fern-spores were called, possessed various miraculous powers. These were touched upon frequently by the early poets. In Shakespeare's "Henry IV" Gadshill exclaims:
"We have the receipt of fern-seed, we walk invisible."
He is met with the rejoinder:
"Nay, I think rather you are more beholden to the night than to fern-seed, for your walking invisible."
One of Ben Jonson's characters expresses the same idea in much the same words:
"I had no medicine, sir, to walk invisible.
No fern-seed in my pocket."
"That spring like fern, that infant weed,
Equivocally without seed,
And have no possible foundation
But merely in th' imagination."
In view of the fact that many ferns bear their spores or "fern-seed" somewhat conspicuously on the lower surfaces of their fronds,
Fig. 8 it seems probable that the "fern" of early writers was our common Brake, the fructification of which is more than usually obscure, its sporangia or "fern-seed" being concealed till full maturity by the reflexed margin of its frond. This plant is, perhaps, the most abundant and conspicuous of English ferns. Miss Pratt believes it to be the "fearn" of the Anglo-Saxons, and says that to its profusion in their neighborhood many towns and hamlets, such as Fearnborough or Farnborough, Farningham, Farnhow, and others owe their titles. The plant is a noticeable and common one also on the Continent.
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.
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.
It is interesting to examine with a good magnifying glass the sporangia borne on the lower surface of a mature fertile frond. In many species each sporangium or spore-case is surrounded with an elastic ring, which at maturity contracts so suddenly as to rupture the spore-case, and cause the expulsion of the numberless spores (Fig. 7).