The prothalli of the adders'-tongues and of the club-mosses are generally tuberous, and grow half buried in the soil, or beneath its surface. Those of the ferns and horsetails are green and leaf-like. But all are alike short-lived; all are quite destitute of woody tissue, and all are very small in comparison to the parent-plant.
The spores shed by the largest of our native ferns develop into prothalli less than half an inch wide in their widest part. They lie pressed close to the surface of the ground, or sometimes beneath it, and being so tiny and so retiring in their habits it is difficult to find them.
A pot in which a fern-plant has come to matur ity and shed its spores, will probably contain some growing prothalli, and we may be able to find them by careful turning over of the surface-soil. But the details of their structure can be studied only by aid of a microscope of four or five hundred diameters.
By use of the lenses we have learned that after the prothallus has "got its growth," two sets of organs appear upon its under surface. These fill the same place in the history of the fern that stamens and pistils do in the history of the flowering plant. They are called antheridia and archegonia.
The first antheridia appear when the prothallus is three or four weeks old. They are most often formed toward the point of the heart, and are scattered over its lower surface, apparently without definite order.
Each antheridium is, at first, a single cell protruding slightly from the surface of the heart, and looking deceptively like a young root-hair.
Grown older, it is a little chamber, with a single layer of cells forming its encompassing wall, and with its interior packed quite full of tiny globes. When the antheridium has reached fullest maturity the cells, which wall in the little chamber, absorb water freely, swell, and burst open.
The minute globes, which have been cribbed, cabined, and confined, are now set free. Each globe is what botanists call a "mother-cell," and coiled up inside it lies something which looks like a strap, with a narrower and a broader end. This is an "antherozoid" (Fig. 69). Soon after the mother-cell comes out of the antheridium it bursts, and the antherozoid, which has been lying in it, curled up and motionless, finds itself thrust forth into the great world.
Fig. 69. - Antherozoids of Pteris serrulata.
(From the Vegetable World).
Being cast upon his own resources has had an energizing and awakening effect upon many a human idler or Jack-a-dreams. And the little antherozoid, turned loose, acts as if it had become instinct with conscious life.
Its coils draw apart, and we see that at the thicker end of the spiral is a tiny drop of jelly, which is all that now remains of the contents of the mother-cell, while at the thinner end there is a dense tuft of fine, curved filaments.
These tremble and sway like the fins of a fish, and by their aid the antherozoid can swim about, with a motion so like that of an aquatic animal that one who watches it is inclined to doubt that it is endowed only with the blind, unconscious life of the vegetable world. Hence, the tiny thing is sometimes called a spermatozooid, for the Greek suffix zooid signifies "like an animal," while sperma means "a germinal principle of life".
There is ample opportunity for the display of its natatorial powers, for to a swimmer so minute every drop of dew is a lake.
While Nature has been giving birth to these little navigators, there have been forming, on the cushion of the prothallus, the archegonia, which are the reasons of their being. An archegonium also begins life as a single cell, on the under surface of the prothallus. A little later a crosswise partition appears, dividing the cell into an upper and a lower portion. More partitions are formed, making a cluster of cells, while the life of the prothallus mould the plastic young tissue till the maturing archegonium takes the shape of a flask, with a proportionately very long and thick neck, curved over to one side (Fig. 70).
The curve is generally in such a direction that the mouth of the flask points toward an antheridium.
At first the flask's mouth is closed, and its neck is filled with a row of cells, called the "neck-canal cells." But a little later these dissolve into mucilage, and at the same time the lips of the flask draw apart. And from the flask's mouth, at this date in its history, there is discharged an acid which is attractive - but we do not yet know just how or why - to the spermatozoids.
Fig. 70. - Young archegonium of a garden maidenhair (Adiantum cuneatum). (Much magnified).
Some rainy day or dewy night, when the under surface of the prothallus is wet, the active little swimmers approach the open neck of the arche-gonium, and are lured into it. And down in the rounded part of the flask they find the "affinity" which they have been unconsciously seeking, a naked globe of colorless jelly known as the "oosphere." One spermatozoid enters the oosphere, and mingles with it, and with this act of fusion the life-purpose of the prothallus is accomplished.
The now fertilized oosphere surrounds itself with a delicate membrane, and becomes the "oospore".
So again in the life-history of the fern we have come around to the single "cell" or globe of protoplasm from which we can trace the development of every living organism.
From the first globe - the fern-spore - creative Nature made the tiny heart-shaped prothallus. From the second globe, - the oospore, - she will make the perfect fern.