Seed dispersal

“ Then a breeze tears off a few pods and they fall on the snow, which has filled up all the crevices in the grass and between the dead leaves and rubbish. Each half-pod, freighted with every other seed, is admirably constructed; like an ice boat, it has a sail always spread to the breeze. In this way there is often nothing to hinder some of the seeds from going a mile or two in a few minutes, now and then striking some object which jars off a seed or two. The seeds are very hard, and no doubt purposely so, that they may not be eaten by insects or birds ”

“ For several days the flowers of a head blossom in succession, each night to be snugly wrapped by the scales, and the next day to be again left open, if the weather be fine. After each flower in turn has been allowed to see the light, and after all have been crawled over by bee and wasp to distribute the yellow pollen that seeds may be produced, there is nothing else to do but patiently wait for a week or two while receiving food from the mother plant to perfect each little fruit and seed. During all this period of maturing, day and night, rain or shine, the scales hold the cluster closely ”

“ Within are a few seeds, hard and smooth. Why are they so hard? Will it not be difficult for such seeds to get moist enough and soft enough to enable them to germinate? The hard coats enable the seeds to remain uninjured for a long time in the water, in case one or two cells of the papery pods are broken open; and after the tough pod has decayed and the seeds have sunken to the moist earth among the sticks and dead leaves, they can have all the time they need for the slow decay of their armor. Sooner or later a tiny plant is likely to appear and produce a beautiful bush. ”

“ No slight breeze can dislodge the seeds, but just see how they behave in a good gale! The elastic stems are swayed back and forth against each other, and some of the upper seeds are tossed out by the wind that passes through the lattice, and at such times are often carried to some distance. The seeds at the top having escaped, the dry pods split down farther and still farther and open still wider, till the bottom is reached. ”

“ There would have been some uniformity in its methods, whereas the modes of dispersal are infinite. The qualities and characters that happen to be connected with dispersal belong to a plant’s development in a particular environment. They can never have been adapted to another set of conditions that lie quite outside that environment. There is a relation of a kind between the specific weight of wood and the density of water, and this, in a sense, sums up the connection between a seed and its distributing agencies. ”

“ The small flowers are borne on a long, elastic, and rather stiff stem, and each flower stalk has a joint just at the base. As this fruit matures, the joint becomes very easy to separate. It dries with a tension, so that, if touched, the fruit goes with a snap and a bound for several feet. The shaking produced by the wind jostling several against each other is sufficient to send off a number of ripe fruits in every direction. Like many other plants we have seen, this has more than one way of scattering seeds, and often more than two ways. ”

“ An advantage in dispersal, very characteristic of the order, is afforded by the seeds, which have a striking peculiarity of germination. While the fruit is still attached to the parent branch the long radicle emerges from the seed and descends rapidly towards the mud, where it may even establish itself before falling off. ”

“ The twisting and untwisting of the awns enable the grain to bore through the fleeces, and even to penetrate the skins and make wounds which sometimes cause the death of the animal. Examine also seeds of pin clover, Alfilerilla, which is becoming abundant in many parts of the world.
33. Winged fruits and seeds fall with a whirl.—The large fruit of the silver maple falls in summer. As these trees are most abundant along the margins of streams, the fruit often drops into the water and is carried down stream to some sand drift or into the mud, where more sand is likely to cover them. ”

“ By fertilizing flowers, by distributing plants, and by other helpful acts, animals pay for at least a portion of the damage they do.
By an almost infinite number of devices, we have seen that seeds and fruits flee from the parental spot on the wings of the wind, float on currents of ocean, lake, and river. They are shot by bursting pods and capsules in every direction. With hooks, barbs, and glands they cling to the covering of animals. ”

“ Confining to a permanent spot invites parasites and other enemies, and a depleted soil, while health and vigor are secured by frequent migrations. The more we study in detail the methods of plant dispersion, the more we shall come to agree with a statement made by Darwin concerning the devices for securing cross-fertilization of flowers, that they "transcend, in an incomparable degree, the contrivances and adaptations which the most fertile imagination of the most imaginative man could suggest with unlimited time at his disposal."6 ”

“ In this way seeds of water plantain, sedges, grasses, rushes, docks, arrowhead, pondweeds, duckweed, cat-tail flag, bur reed, bladderwort, water crowfoot, and many others are transported from one pond, lake, or stream, to another. In some cases enough of a living plant may be detached and carried away to keep on growing. Darwin found on the feet of some birds six and three-quarter ounces of mud, in which were five hundred and thirty-seven seeds that germinated. ”

“ In pursuing your way you rub off a portion of these fruits, and at the end of the journey, or before, you sit down in some comfortable spot and deliberately pick off the unwelcome stick-tights. At such times you have been the means of transporting seeds, and you have left them scattered about ready to grow. If you ever were so fortunate as to live on a farm, you must have seen your father or his hired help carefully look about the field or the wood lot and remove all the bur-bearing plants that could be found before turning in his flock of sheep or the colts and cattle ”

“ As animals starve out, in certain seasons when food is scarce, or more likely migrate to regions which can afford food, so plants desert worn-out land and seek fresh fields. As animals retreat to secluded and isolated spots to escape their enemies, so, likewise, many plants accomplish the same thing by sending out scouts in all directions to find the best places; these scouts, it is needless to say, are seeds, and when they have found a good place, they occupy it, without waiting for further instructions. ”

“ When the rivers break up, the seeds are carried down stream, and perhaps left to grow on dry land after the water has retired. Most of the commonest plants, the seeds of which are usually transported by water, are insignificant in appearance and without common names, or with names that are not well understood. ”

“ However, the spreading is not always a slow, creeping process, for sometimes these low plants spread over an incredible distance in a short space of time. In some instances they appear suddenly almost anywhere, and at any season of the year. ”

“ By the time the quick hot fire has passed over, the cones have slowly opened and begun scattering seeds on the vacant and newly burned ground, at a time when there is the best possible chance for them to grow. ”

“ This they do simply in order to disseminate their kind in new and unoccupied spots, where the seedlings will root and find an opening in life for themselves. Observe, indeed, that the very word 'disseminate' implies a general vague recognition of this principle of plant-life on the part of humanity. It means, etymologically, to scatter seed; and it points to the fact that everywhere in nature seeds are scattered broadcast, infinite pains being taken by the mother-plant for their general diffusion over wide areas of woodland, plain, or prairie. ”

“ Summer preparation for grass gives opportunity to destroy a great part of the seeds in the surface of the ground, and it is only when they are near the surface that the seeds of most weeds will germinate. Deep harrowings, continued up to time of planting, not only rob land of water, but they bring to the surface new lots of seed that had been safely buried, and become a part of the actual seeding when the grass, clover, or alfalfa is sown. ”

“ Even in such cases partial or total destruction of the seed often results from birds and rodents. In exposed situations where the soil is shallow, or where because of climatic conditions soil dries out several inches deep during the growing season, the seed may not germinate at all, or the young seedlings may be killed before they have time to send their roots down to the permanent moisture level. In such situations, planting is the only reliable method. ”

“ Seeds may lie dormant for many years and then vegetate, while they endure extremes of heat, of cold, of drought, or of moisture which would almost always be fatal to animal germs. Among the causes of the dispersal of seeds De Candolle enumerates the wind, rivers, ocean currents, icebergs, birds and other animals, and human agency. Great numbers of seeds are specially adapted for transport by one or other of these agencies. Many are very light, and have winged appendages, pappus, or down, which enable them to be carried enormous distances. ”

“ These grounds it is able to hold against all comers, however big and strong, by saving its seeds until death, when all it has produced are scattered over the bare cleared ground, and a new generation quickly springs out of the ashes. Thus the curious fact that all the trees of extensive groves and belts are of the same age is accounted for, and their slender habit; for the lavish abundance of seed sown at the same time makes a crowded growth, and the seedlings with an even start rush up in a hurried race for light and life. ”

“ Dispersal of Seeds and Fruits serves (a) to scatter these reproductive structures and so reduce internecine competition; (b) to bring the seeds into new surroundings, which may be more favourable than those of the parent plant. The chief agents of dispersal are wind and animals. Very minute seeds, like those of orchids, are carried away by the gentlest air-currents. Larger wind-borne seeds are winged, as in the pine, most Bignoniaceæ, &c.; or provided with a tuft of hairs acting as a parachute, as in willow, willow-herb, cotton, &c. ”

“ There must, as in the wind-carried pollen, be a tremendous wastage, yet sufficient quantities of it do fertilize the females, particularly in the ditch grass, which fruits very freely.
Whether it be any of the various contrivances for insect fertilization, or by the winds, or, as in the eelgrass, by the water, the climax of the flower’s life is always reached in this act. For all annuals the plants, also, begin to die down then, a process that is completed with the production of seed, which is, of course, the object of all those varied modes of fertilization. ”

“ If you want to keep moisture out of the soil, there is no way of doing it more effectually than by allowing it to become crusted over. But if you want to get all possible moisture into it, keep it light and porous. Such a condition will make it possible for it to absorb whatever moisture there may be in the air.
Make it a rule to go over your plants when they come into bloom and cut off every faded flower, to prevent the formation of seed. Most plants will give but one general flowering period if left to manage their own affairs. All their energies will be expended in the production of seed. ”

“ Annuals persist through the winter season, only in the form of seed, so that a single wet summer such as the last (1909) , by interfering with the ripening of the seeds, might threaten with extinction an entire species. Annuals have but little food stored up for a sudden outburst of activity on the approach of spring, nor can the tiny root of a budding seedling obtain from the soil all that is necessary for the rapid production of flowers and seeds. ”

“ It is poor practice to sow any kind of small seeds before a rain that seems imminent. If it forms a crust, or causes weed-seed germination along with that of the grass seeds, only harm results. When seeds are put into a dry soil, and a light shower comes, there may be germination without sufficient moisture to continue life in the plants. ”

“ Some of it falls on the roadway; some of it is snatched away by the birds; some of it is caught among the bushes. Yet the sower proceeds in no niggardly fashion. He strides away across the field scattering the seed broadcast, far beyond the border where he expects a crop, for he knows that, though much shall be wasted, whatever seed may fall on good ground will have miraculous increase. There may be prodigality of waste, but there shall be prodigality of reproduction. If but one seed in thirty takes root in good soil it may produce thirty or sixty or a hundred fold. ”

“ This is due for the most part to the inability of the large plants to obtain sufficient water for seed production. The much smaller plants of the second crop do not require as much moisture as the larger plants of the first crop, as the vegetative growth is seldom more than half as much. ”

“ The element of hazard, therefore, is too great in northerly areas to justify sowing the seed thus. But on the bench lands of the mountain valleys there may be instances in which the seed may be sown so late in the autumn that it will not sprout before winter sets in, but lies in the soil ready to utilize the moisture, so all important in those areas, as soon as the earliest growth begins in the spring. ”