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The Dispersion of Seeds
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The Dispersion of Seeds
PLINY, whose work embodies the natural science of his time, tells us that some trees bear no seed. âThe only ones,â says he, âamong the trees that bear nothing whatever, not so much as any seed even, are the tamarisk, which is used only for making brooms, the poplar, the Atinian elm [Ulmus campestris rarely does in England], and the alaternusâ; and he adds that âthese trees are regarded as sinister [or unhappy, infelices] and are considered inauspicious.â
As there is even yet a lingering doubt in many minds with regard to some trees, whether they bear flowers and seed or not, it is the more important to show not only that they do, but for what purpose.
We are so accustomed to see another forest spring up immediately, as a matter of course, when one is cut down (whether from the stump or from the seed), never troubling ourselves about the succession, that we hardly associate seeds with trees, and do not anticipate the time when this regular succession will cease and we shall be obliged to plant, as they do in all old countries. The planters of Europe must therefore have a different and much more correct notion of the value of seeds than we. To speak generally, they know that forest trees spring from seeds; but we know only that they come out of the earth when we cut them down, as regularly as the fur grows on the hides of animals after the summer has thinned it. As time elapses and the resources from which our forests have been supplied fail, we too shall of necessity be more and more convinced of the significance of the seed.
My purpose in this chapter is to show how, according to my observation, our forest trees and other vegetables are planted by Nature.
When, hereabouts, a forest springs up naturally where none of its kind grew before, I do not hesitate to say that it came from seeds. Of the various ways by which trees are known to be propagatedâby transplanting, cuttings, and the likeâthis is the only supposable one under these circumstances. No such forest has ever been known to spring from anything else. If anyone asserts that it sprang from something else, or from nothing, the burden of proof lies with him.
It remains, then, only to show how the seed is transported from where it grows to where it is planted. This is done chiefly by the agency of the wind, water, and animals. The lighter seeds, as those of pines and maples, are transported chiefly by wind and water; the heavier, as acorns and nuts, by animals.
To begin with the pitch pine. All my readers probably are acquainted with its rigid conical fruit, scarcely to be plucked without a knifeâso hard and short that it is a pretty good substitute for a stone. Indeed, this is the use to which the Romans once put their kind. They called it the pine nut, and sometimes the apple of the pine, whence pine-apple. It is related that âwhen Vatinius gave a show of gladiators to conciliate the people, by whom he was much hated, they pelted him with stones. The ediles made an order forbidding the people to throw anything but apples within the arena; and on this the people pelted Vatinius with the apples of the pine tree. The question was, then, whether this was to be considered as a defiance of the law; and the celebrated lawyer Cascellius being consulted, replied, âThe pine nut, if you throw it at Vatinius, is an apple.ââ
If not plucked, these cones hold on all winter and often many years. You see old gray cones, sometimes a circle of them (which the Romans appear to have called azaniae) within two feet of the ground on the trunks of large trees, having been formed there when the tree was young twenty or thirty years beforeâso persistent are they.
Within this strong, prickly, and pitchy chest are contained about a hundred dark brown seeds in pairs, each pair occupying a separate apartment behind its prickly shield. A very thin membrane or wing about three-fourths of an inch long extends from one end of each seed, which it clasps in its divided extremity like a caged bird holding the seed in its bill and waiting till it shall be released that it may fly away with and plant it.
For already some rumor of the wind has penetrated to this cell, and preparation has been made to meet and use it. According to Darwin, Alphonse De Candolle has remarked that winged seeds are never found in fruits which do not open. They were designed for flight. This wing is so independent of the seed that you can take the latter out and spring it in again, as you do a watch crystal.
The sun and wind, which have the key to these apartments, begin to unlock them with a crackling sound in the second or third fall and continue to do so here and there all winter long, and there they lie exposed with their thin, curved handles upward and outward to the wind, which ever and anon extracts them and conveys them away. If they chance to be released in calm weather, they fall directly to the earth, rapidly whirling all the way; but if there is any wind, they are borne more or less to one side. They remind me most, after all, of some deep-bellied fishâan alewife or shadâwith their flanks and a tail curving to this side or that, the whole of whose flexible body is a sort of wing or fin fitted not for the varied and prolonged flight of birds, but to steer and assist its course in the stronger or grosser current in which it floatsâschools of brown fishes which perform this short migration annually.
Nature always adopts the simplest modes which will accomplish her end. If she wishes one seed to fall only a little to one side from a perpendicular line, and so disseminate its kind, perhaps she merely flattens it into a thin-edged diskâwith some inequalityâso that it will âscaleâ a little as it descends. In course of time, when it contemplates a more distant and wider flight from the pine-tree tops to the earth, moveable edges, called fins or wings, may be added to this simple form.
The pitch pine is a very seedy tree and is peculiarly bent on extending its domains. It begins to bear when very smallâsometimes when less than two feet high.
I have noticed that where, on account of the poverty or rockiness of the soil, these trees find it hard to live at all, they bear the more fruit. I have counted on a solitary pitch pine, which was only three feet high and as broad as high, spreading flat over a rock on a hilltop, more than one hundred cones of different ages. Having scaled this rocky citadel, its first care, though in a crippled state, was to call a hundred followers about it and so get undisputed possession.
Michaux observes that âwherever these trees grow in masses, the cones are dispersed singly over the branches, and . . . they release the seeds the first autumn after their maturity; but on solitary stocks, the cones are collected in groups of four, five, or even a larger number, and remain closed for several years.â
Not only is it the outside trees that bear the most seed, where it is most required, but only a considerable wind, which can transport the seed to a distance, is able commonly to set it free, so that it does not fall at once to the ground, where it would be wasted. All have noticed the dense groves of pitch pine of uniform height, which were perhaps planted in a single gale, and you can often tell what tree the seed came from. In my mindâs eye, and sometimes partially with my bodily eye, I see the seeds from which they sprang falling in a dense shower which reaches twenty or thirty rods on one side, like grain scattered by the hand of the sower.
Sometimes a man cuts off a lot of young pitch pines, leaving only the old parent tree to seed the ground again. They are not commonly noticed till half a dozen years old.
As I went by a pitch-pine wood the other day, I saw a few little ones springing up in the pasture from these seeds which had been blown from the wood. There was a puny one which came from the seed this year, just noticeable in the sod, and I came near mistaking it for a single sprig of moss. It was, as it were, a little green star with many rays, half an inch in diameter, lifted an inch and a half above the ground on a slender stem. What a feeble beginning for so long-lived a tree! By the next year it will be a star of greater magnitude, and in a few years, if not disturbed, these seedlings will alter the face of Nature here. How ominous the presence of these moss-like stars to the grass, heralding its doom! Thus, from pasture this portion of the earthâs surface becomes forestâbecause the seeds of the pine, and not of moss and grass alone, fell on it. These which are now mistaken for mosses in the grass will perhaps become lofty trees and endure two hundred years.
Pitch-pine seeds
Unlike the white pine, the pitch pine is opening its cones and dispersing its seed gradually all winter, and it is not only blown far through the air, but slides yet further over the snow and ice. It has often occurred to me that it was one value of a level surface of snow, especially a crusted snow, that by its smoothness it favored the distribution of such seeds as fell on it. I have many times measured the direct distance on a snowy field from the outmost pine seed to the nearest pine to windward, and found it equal to the breadth of the widest pasture. I have seen that the seed thus crossed one of our ponds, which is half a mile wide, and I see no reason why it should not be blown many miles in some cases. In the fall it would be detained by the grass, weeds, and bushes, but the snow having first come to cover up all and make a level surface, the restless pine seeds go dashing over it like an Esquimaux sledge with an invisible team until, losing their wings or meeting with some insuperable obstacle, they lie down once for all, perchance to rise up pines. Nature has her annual sledding to do, as well as we. In a region of snow and ice like ours, this tree can be gradually spread thus from one side of the continent to the other.
By the middle of July, I notice on the shore of the above-mentioned pond, just below the high-water line, many little pitch pines which have just sprung up amid the stones and sand and muck, whose seed has been blown or drifted across. There are some places for a row of pines along the waterâs edge, which at length, after fifteen or twenty years, are tipped over and destroyed by the heaving of the frozen shore.
I noticed lately a little pitch pine which had come up on the sandy railroad embankment in our meadows, just sixty rods by pacing from the nearest pine tree; nor is this uncommon. I have seen a single pitch pine spring up spontaneously in my yard in this village, about half a mile from the nearest of its kind, with a river and its deep valley, and several roads and fences between, and it grows there yet. This tree would soon sow itself in all our yards if they were neglected.
Every year the seed of the pines is blowing thus from our pine woods and falling on all sorts of ground, favorable and unfavorable. When the circumstances are propitious, a forest of pines springs up, especially if the land to leeward is open or has been lately cleared, or plowed, or burned over.
One man accordingly tells me, and there are countless cases like it, that he had a lot of pines, which being cut, shrub oaks came up. He cut and burned and sowed rye, and, it being surrounded by pine woods on three sides, the next year a dense growth of pines filled the ground.
Squirrels also help to disperse the pitch-pine seed. I notice every fall, especially about the middle of October, a great many pitch-pine twigs or plumes, which have evidently just been gnawed off and left under the trees. They are from one-half to three-fourths of an inch in thickness, and often have three or four branches. I counted this year twenty under one tree, and they were to be seen in all pitch-pine woods. It is plainly the work of squirrels. As I had not chanced to detect their object, I resolved last fall to look into the matter.
Accordingly, thinking it over one night, I said to myself, âAnything so universal and regular, observed wherever the larger squirrels and pitch pines are found, cannot be the result of an accident or freak, but must be connected with the necessities of the animal.â I have found that the necessaries of my life are food, clothing, shelter, and fuel; but the squirrels use only food and shelter. I never see these twigs used in constructing their nests; hence, I presume that their motive was to obtain food, and as its seeds are all of the pitch pine that I know them to eat, my swift conclusion was that they cut off these twigs in order to come at the cones, and also to make them more portable. I had no sooner thought this out than I as good as knew it.
A few days after, as I was passing through a pitch-pine wood, where as usual, the ground was strewn with twigs, I observed one eleven inches long and about half an inch thick, cut off close below two closed cones, the stem of one cone also being partly cut. Also in open land three or four rods from this grove, I saw three twigs which had been dropped near together. One was just two feet long and cut off more than a foot below three cones which were still left on it, two on one branch and one on another. One of the others was longer still.
Thus, my theory was confirmed by observation. The squirrels were carrying off these pine boughs with their fruit to a more convenient place either to eat at once or store up. You would be surprised to see what great limbs they carry, and to what distances sometimes. They are stronger than we suppose. A neighbor tells me that he had a gray squirrel which would take a whole and large ear of corn, run out of a broken window of his corn barn and up the side and over the roof, or perhaps high into an elm with it.
Most of the twigs which you see in the woods, however, are smaller, mere single plumes, which have been sheared off above the cones, whether to lighten the load or to come at the cone. They were so generally cut off last fall, when comparatively few pitch pines bore any cones, that I commonly detected the fertile trees when going through a grove by seeing these green twigs strewn on the brown ground beneath them.
It is surprising how rudely the squirrels strip and spoil the trees on which they depend. I often thought what a hue and cry I should have made about it, if they had been orchard trees and belonged to me. Even the pitch pines thus get trimmed after a fashion, and perhaps for their benefit.
In most cases, evidently, they carry off the cone alone; but perhaps a strong squirrel prefers to carry three cones, twig and all, at one trip than come three times after single cones. I frequently see where they have dropped the bare cones, having been interrupted, and I once counted twenty-four such cones quite fresh and unopened brought together under a solitary pine in a field, evidently ready to be transported to another place.
It chanced that I did not see last October where any of these cones had been eaten or stripped, as I often do. I conclude therefore that most of them must have been collected into holes in trees or in the earth, where the squirrels live, and possibly some are buried singly as nuts are.
Think how busy the squirrels are in October, in every pitch-pine grove all over the state, cutting off the twigs and collecting the cones. While the farmer is digging his potatoes and gathering his corn, he little thinks of this harvest of pine cones which the squirrel is gathering in the neighboring woods still more sedulously than himself.
In this way even the squirrels may spread the pine seed far over the field. I frequently see a pitch-pine cone far out in an open field, where it was dropped by a squirrel when on its way toward some tree or wall or stumpâor oftener by the side of a fence on which the squirrel travelled at a considerable distance from the woods; and there it will sometimes lie covered by the snow all winter, and not expand and shed its seeds till the snow goes off and it feels the heat of the sun.
The pitch-pine cones have very stout and tough stems, the woody part alone being often a quarter of an inch in diameter; and they are scarcely so long as wide, which makes them hard to come at. But rigid and unmanageable as they are, almost every fresh cone of this kind which you see on the ground was cut off by a squirrel, and you can plainly see the marks of its teeth on the stem. He cuts it as he bends it, and a very few cuts suffice to separate it from the branch.
When he has thus plucked it, sitting on a fence-post or other perch, he begins at the base of the cone and gnaws off the scales one after another, devouring the seeds as he goes, leaving only half a dozen empty scales at the extremity. The close-shaven cone presents thus a pretty flower-like figure, which it would take a long time to produce with a knife.
This plucking and stripping a pine cone is a business which he and his family understand perfectly. It...
