Two sorts of selaginella are cultivated under the name of "lycopodium," and may be seen draping the stages in greenhouses, or making a moss-like mat all over the floor in florists' windows. These plants bear spores of two sorts and sizes, which ripen at about the same time, and fall to the ground together (Fig. 82). Then the substance contained in each of the smaller spores develops into a tiny "male" prothallus, consisting of one vegetative cell, and beside it, a little chamber full of spermatozoids. And in the larger spore meantime there is formed a "female" prothallus, which soon grows big enough to rupture the spore-wall and protrudes through the fissure it has made.
Fig. 82. - A larger ("macro") and two smaller ("micro") spores of Selaginella martensii (the Lycopodium stoloniferum of florists). All magnified alike to show actual comparative size.
In this protruding part, archegonia, like those of the fern-prothallus, are formed, and at about the same time the spermatozoids in the smaller spore are ready to seek their affinities, and the spore-wall bursts, setting them free. They are provided with little cilia, which quiver and vibrate like the cilia on the fern-spermatozoid, and by means of these they swim actively over the dewy- or rain-soaked ground, till they find the larger spore and in it the archegonium they have been seeking.
So the prothalli of the selaginella are male and female, and they spend their brief lives attached to their respective spores, and almost completely-enclosed by them.
The gymnosperm represents the next phase of plant-history. For the sturdy pine-tree, like the little moss-like selaginella, has two prothallus parents, both tiny and short-lived. The selaginella prothallus which lives in the smaller or "micro"-spore is minute, though the one which develops in the larger or "macro"-spore comes a little way out into the world, and in some species grows large enough to be seen with a pocket-lens. But neither of the prothallus parents of the pine can be seen without the aid of a high-power microscope, and they never leave the spores in which their lives begin.
The larger or female prothallus is completely enclosed by the macrospore, and the macrospore is developed inside the ovule, and stays there during its entire life. This prothallus is only a little mass of cell-tissue, almost colorless, because it lives in the dark. After it has "got its growth" tiny archegonia are formed in it, and these stand in such a position that their flask-mouths open toward that spot in the ovule which is not covered by the seed-coat. From three to five of these little flasks are prepared for pollen in each of the pine-ovules.
At the season when the winds are freighted with the pollen of the cone-bearers the scales of the pistillate flower draw apart, so that the precious dust can slip down between them to the ovule. And just at this stage of affairs a tiny drop of fluid exudes from the opening in the ovule's coat. The golden grains brought by the breezes are caught and held in this, and as the fluid evaporates, or is absorbed, they are gradually drawn down to the ovule's surface.
The ripe pollen-grain of the pine is not a mere bag of jelly, as is the pollen-grain of the crocus. It has two compartments or "cells" like the smaller spore of the selaginella. One of these is merely vegetative, and one, a little later, develops into the pollen-tube. The tube put forth by the crocus pollen grain contains one globule of vitalizing protoplasm, the "generative cell." But the perfected pollen-tube of the pine contains two generative cells.
The tube penetrates the tissue of the ovule for a very short distance, and then there is a pause, while the little archegonia down below are coming to maturity. After this interval of rest, the tube makes its way to the macrospore, pierces its delicate wall, and enters the neck of an archegonium. Here it finds a little globule of protoplasm, similar to that which is contained by the archegonium of a fern. One of the generative cells from the pollen-tube fuses with this globule, and after their union is complete creative life begins to mould their combined substance into a little cone-bearer.
The second "generative cell" of the pine-pollen seems to be a mere understudy. It comes down the pollen-tube into the ovule, but the most recent investigators agree that it does not enter an archegonium, and that, after a brief interval it melts away, as it were, and disappears. It may be a reminiscence of a now obsolete method of fertilization practiced by the pine's ancestors.
Fern, selaginella, pine, and rose seem to represent successive stages in the dwindling of the pro-thallus. That of the fern is a perfect plant, with green leafy surface and serviceable root-hairs. It comes out into the great world, supports itself independently there, and reaches, sometimes, the ripe age of two years. That of the selaginella is minute, and spends its brief life almost encompassed by the spore. That of the pine is highly microscopic, and never leaves the spore, but continues utterly dependent upon the parent-tree so long as it lives. And careful investigation and comparison show in the highest flowering plants the last vestiges of the prothallus, here almost obliterated, but still distinct enough to show the far-off kinship of fern and rose.
A few years ago naturalists believed that the ovule of the flowering plants was quickened by union with a globule of protoplasm from the pollen-tube, while the female cell of the higher flower-less plant developed at the vitalizing contact of a spermatozoid, and that here lay the great difference between the patricians and the plebeians of the vegetable world.
But recently Mr. Herbert Webber has studied the whole process of fertilization in a subtropical gymnosperm, the coontie or arrow-root of southern Florida (Zamia integrifolia).
His investigation has proved that the kinship between the flowering and the flowerless plants is far closer than has been hitherto supposed. For what goes down through the pollen-tube of the coontie-blossom is not a mere globule of jelly, as in the crocus, or two globules of jelly, as in the pine, but two peg-top-shaped spermatozoids, a-quiver with cilia, and spinning around as if instinct with conscious life. And two Japanese botanists have found spermatozoids in the pollen-tubes of two other cousins of the pines, the graceful cycad and the gingko-tree.