We have seen that some of our familiar trees seem to be passing through changes in the structure and mode of fertilization of their flowers. Others are even now diminishing the number of their seeds. Nature, keeping up an age-old habit, forms a large number of germs; but the trees, having adopted a newer habit, neglect most of these germs, and bring only a remnant of them to maturity. But these comparatively few offspring are sent into the world better nourished, better provided for, better equipped for the battle of life than they would have been had the parent tree undertaken the maintenance of a larger number of descendants, and thus they profit by the fate of their little brothers which perished untimely.
The horse-chestnut blossom has a three-celled ovary, with two ovules in each cell; but the ripe horse-chestnut bur never holds more than three nuts, and sometimes only two, or even a solitary one. "Yet the vestiges of the seeds which have not matured," says Prof. Gray, "and of the wanting cells of the pod, may always be detected in the ripe pod." The very young acorn is divided into three compartments, and each compartment has two ovules hanging from its summit. One might, therefore, expect the mature acorn to be a husk enclosing six small nuts or seeds. But, in fact, five of the cells are all but obliterated in the forming fruit, which thus becomes one-celled and one-seeded.
Very young horse-chestnut bur cut crosswise (a) and lengthwise (b), showing that it is at this stage a three-chambered pod enclosing six seeds.
Very young acorn cut crosswise (c), showing its three chambers and six ovules. Older acorn cut crosswise (d), showing that two of the original six ovules have vanished and that a third is dwindling.
Fig. 14. - Young horse-chestnut bur and young acorns.
But if we strip the shell off a mature acorn we can generally see near its base three irregular lobes, which are a reminiscence of the three chambers of the young ovary.
And between these lobes are the last vestiges of the partitions which once completely trisected the baby-acorn.
The pistil of the maple blossom is a double affair, with two styles, two stigmas, two ovaries, and four ovules, two in each ovary; but the winged twin-fruit which results from its development contains but two seeds (Fig. 15).
It is not unusual for atrophy to go still further and for one-half of the double fruit to stop growing very early in the season, so that in the end the fruit turns out to be an unsymmetrical thing, with one side swelled into firmer and plumper proportions, because nourishment has been withheld from the other.
In the acorns and horse-chestnuts which come to maturity, the baby-plant is supplied with a particularly rich and plentiful stock of starches on which to feed while it does its first growing, and it is protected from damp and from insect enemies by a tough, horny shell. The maple germ is also provided with sustenance for its first days of life, is wrapped in a strong covering, and is provided with a wing, so that it can fly far before the autumn gales. When the descendants of these trees are so well started, a large proportion of them will survive, and thus the oak, horse-chestnut, and maple families are quite as well kept up as are the families of other trees, which cast to the winds a large number of seeds less fully equipped for the battle of existence.
Fig. 15. - Twinned fruit of the maple.
(From the Vegetable World).
For when a plant, in shiftless and stepmotherly fashion, hands its offspring over to those untender nurses, luck and chance, it follows that an enormous proportion of the offspring will die.
By investigating the blossoms of the oak, horse-chestnut, and maple, we see that these trees, ages ago, bore very many seeds, which must have received but a scant provision apiece wherewith to start themselves in life. Under these circumstances, the majority of the seedlings would die young, giving the parent-plant the expense of putting an enormous family out into the world, and all to little purpose. To-day, evolution is teaching them "a more excellent way.
"It is a fatal habit," says Grant Allen, "to picture evolution to one's self as a closed chapter. We should think of it rather as a chapter that goes on writing itself for ever. Our fields are full of degenerate flowers which retain some memorial of their old estate, pointing backward, like the fasces of the Byzantine emperors, to the past glories of their race in earlier times." They are also full of plants which bear somewhere about them half-obliterated traces which tell the story of their progress from a lower to a higher form of life.