A prothallus may form a number of archegonia before a spermotozoid finds its way into any one of them. But as soon as an archegonium is fertilized no new ones appear, and the remaining life of the prothallus is expended in nourishing the oospore. Sometimes it so happens that several sister oospores are ready to grow at the same time. But generally in this case one dominates the rest in the great struggle for life, and draws into itself all the nutriment which the prothallus can bestow. And so a prothallus seldom gives rise to more than one little fern.
If a prothallus is insufficiently nourished it may bear antheridia only, and no archegionia at all. Such "male" prothalli are apt to develop after an eccentric fashion of their own. They are often long and narrow, sometimes almost filamentous in form and grow into irregular projections. In fact, they are sometimes "all bubukles, whelks, and knobs," like Bardolph's countenance.
They are relatively small, and may even be reduced to a single vegetative cell, an antheridium, and a few root-hairs.
Several common native ferns, notably the great Osmundas, always give origin to a number of starveling "male" prothalli, in addition to the larger and more symmetrical ones which bear both antheridia and archegonia.
And in a few flowerless aquatic plants, closely akin to ferns, all the prothalli are either male or female. In a few fern-allies the prothallus - male or female as the case may be - is minute and colorless, and remains throughout its brief life partially enclosed within the spore from which it grew. From such plants as this there is but a short upward step to the cone-bearing trees.
But all our familiar ferns of wood, rock, and roadside, the "Filices" of the working botanist become parents of prothalli which escape from the spore in their earliest youth, and live thereafter as independent plants, growing on the surface of the earth, and getting their own honest living by aid of a working-outfit of chlorophyll and root-hairs.
So there is in ferns a true alternation of generations. The fern gives birth to a prothallus, and the prothallus gives birth to a fern. In this curious genealogy there is no resemblance between parent and offspring, but the offspring is a young copy of its grandparent.
The fern prothallus corresponds to a small fraction of the blossom in a flowering-plant. To prove this it would be necessary to plunge so deeply into structural botany that the reader might find the comparison, like many another, odious.
The life-story of the prothallus resembles that of the flower in these respects, that it lives to accomplish one purpose, - that this purpose is the fusion of two reproductive cells, one male and the other female, - and that when this life-work is finished it dies. And as the unfertilized flower lives long past its normal time of blooming, still waiting for breeze or insect to bring it pollen wherewith to set its seed, the unfertilized prothallus may continue to grow for several months, or even, in the case of the Osmunda, for years.
But as soon as spermatozoids have entered the archegonia, and one or two oospores have been formed, the prothallus begins to wither.
The oospore is soon cut into two parts by a vertical partition, and then into four by a horizontal one. Three of these divisions become the stem, leaf and first root of the young fern. The fourth becomes an organ termed "the foot" by means of which the fern draws its support from the parent prothallus till it is old enough to shift for itself. By that time the prothallus is quite depleted and exhausted.
After the fern has passed its earliest youth the first-formed or "primary" root withers away.
In most native species the main stem lies horizontally along the" surface of the earth, or just beneath it. The leaves or "fronds" spring from its upper side, and a number of small, branching rootlets arise, without regularity or system, from its lower surface. Sometimes the half-buried main stem, or " rootstock," is many inches long, and at one end of it there is a large actively-dividing cell - the growing point. But in the tropical tree-ferns the main stem stands erect, and the "growing point" is at its tip-top.
When our native ferns appear above ground in spring, their leaves, or fronds, are rolled downward from the tips like croziers, and by this token we can distinguish them from their near kindred, the adder's-tongues (Ophioglossaceae), which enter the world upright.
The roly-poly ferns of early spring are generally hairy or scaly (Fig. 71), with brown transparent outgrowths which help to protect the tender frond from cold snaps and bitter winds. Later in the season hairs and chaffy scales may still be seen clinging to the fern-stalk and sometimes almost covering its lower portion. Under the microscope these scales are seen to consist each of a single layer of cells, with thickened brown walls, through which a mucilaginous or resinous liquid oozes. Gold and silver ferns have their under surfaces covered with hairs which exude resinous and waxy substances.
Fig. 71. - "Male-fern" (Aspidium felix-maas), showing the prostrate root-stock and the downward roll and scaly covering of the young fronds, (From the Vegetable World).
But the trick of developing hairs is best understood by the tree-ferns, whose young leaves are completely buried in a brown mass of vegetable fur, sometimes utilized by robber man for stuffing matresses.
By latter July most native ferns have attained maturity, and on the backs of the fronds, in many species, we can see dots and dashes of silver-green, dark-green, or brown. These are "sori," and their general plan can be readily seen with a pocket-lens.
Typically each sorus consists of a little scale or lid, covering a group, or perhaps two groups, of stalked sporangia, and each sporangium is a little round or watch-shaped box filled with spores (Fig. 72). The sporangia of many ferns are nearly surrounded by an incomplete ring of large cells, whose brownish walls are of a substance akin to cork. As the sporangium grows older the outer walls of these cells dry and shrink; and as this shrinking proceeds, the incomplete ring begins to straighten itself out. By so doing it pulls upon the surrounding tissue and ruptures the sporangium, scattering the dust-like spores to the four winds. The sporangia of the great Osmundas have no encompassing ring, but they are split by the action of a little group of corky cells, which shrink together as they grow old, and thus first strain and then rend the neighboring tissue.
The spores of native outdoor-ferns remain dormant through the winter and grow into prothalli in the spring.
The Hartford climbing-fern, the common sensitive-fern (Fig. 73), and a few others have instituted a division of labor by which some fronds fulfil all the offices of foliage, while others, which are curiously contracted, produce sporangia and do nothing else. The spore-bearing fronds of the sensitive-fern, browned and desiccated by the winter storms, are conspicuous amid the tender greenery of low-lying fields in early spring. The royal Osmunda, called "flowering-fern" also practices division of labor, but practices it less completely, for the lower part of the great frond is green and leaf-like, while the upper portion is a plumy mass of densely-crowded sporangia.
Fig. 72. - Opening sporangium of a Florida fern (Pteris cretica). (Much magnified).
Fig. 73. - Vegetative and spore-bearing fronds of the sensitive fern (Onoclea sensibilis).
The development of these sporangia begins in early spring, before the fronds unroll, and they attain their full growth by the first of June. So the royal Osmunda takes more than a month's precedence of less methodical ferns, which make all fronds serve both purposes.
The sporangium in all the true ferns is formed from a single superficial cell. This cell grows so as to project above the general surface of the frond, and when it is hemispherical it is cut in two by a crosswise partition.
The inner section will become the stalk of the sporangium, and the rounded outer portion will eventually be fashioned into the sporangium itself.
But in the adder's-tongues and some other fern-allies the sporangia are developed each from a little cluster of superficial cells.
Upon this difference botany divides the ferns and their nearest of kin into two great groups.
The adder's-tongues (Ophioglossaceae) belong to the smaller and older of these groups - the Euspor-angiateae. They are feeble descendants of a very ancient and once powerful and numerous family and are distantly related to the great ferns of the coal measures, which were also Eusporangiateae.
But the majority of our native ferns are not, as is so often asserted, the depauperate progeny of a doughty race. They are "Leptosporangiateae," and form their sporangia each from a single cell. This is the more modern method, and is followed by the younger branch of the fern family (Filicinae).
The great majority of our native ferns belong to a younger branch of this younger branch, the Poly-podiaceae, which, as we know from the testimony of the rocks, did not make their appearance till within comparatively recent times. They have multiplied and have taken possession of the land, setting aside the law of primogeniture as Jacob did of old.
The disinherited Eusporangiateae are represented in our country only by the moonworts and the adder's-tongue (Ophioglossum vulgatum). There are but seven species in all, and their largest and most important member is barely two feet tall, while sixty-five species of the Leptosporangiateae are found north of the southern boundary of Virginia, and even in Canadian forest-clearings some of them grow breast-high.
As we go southward the Leptosporangiateae increase in number and in size, till in tropical woods the tall shafts of the tree-ferns rise like the columns of a great cathedral and the long fronds curve upward from their tops like springing gothic arches. One who has seen these truly "cathedral woods" is quite disabused of the prevalent but mistaken notion that the fern family as a whole has "fallen on evil days" (Fig. 74).
Fig. 74. - Spores of a club-moss (Lycopodium complanatum).