Loe raamatut: «Seed Dispersal», lehekülg 5

CHAPTER VII
PLANTS THAT ARE CARRIED BY ANIMALS

With the frosts of autumn ripe acorns, beechnuts, bitternuts, butternuts, chestnuts, hickory nuts, hazelnuts, and walnuts are severed from the parent bush or tree and fall to the ground among the leaves.

37. Squirrels leave nuts in queer places and plant some of them. —Even before the arrival of frosts many of these are dropped by the aid of squirrels, gray and red, which cut the stems with their teeth. The leaves, with the help of the shifting winds, gently cover the fruit, or some portions of it, and make the best kind of protection from dry air and severe cold; and they come just in the nick of time. Dame Nature is generous. She produces an abundance; enough to seed the earth and enough to feed the squirrels, birds, and some other animals. The squirrels eat many nuts, but I have seen them carry a portion for some distance in several directions, and plant one or two or three in a place, covering them well with soil. It may be the thought of the squirrel – I cannot read his thoughts – to return at some future time of need, as he often does. But in some cases he forgets the locality, or does not return because he has stored up more than he needs; or in some cases the squirrels leave that locality or are killed; in any such case the planted nuts are not disturbed. At all events, some of the nuts – one now and then is all that is needed – are allowed to remain where planted. In this way the squirrel is a benefit to the trees and pays for the nuts he eats. He has not lived in vain, for he is a tree planter and believes in arboriculture. His arbor day comes in autumn, and he needs no message from the governor to stimulate him to work.

After some red squirrels had been given black walnuts, a member of my family saw them hide the nuts in all conceivable places, and in some instances place them above a cluster of small branches of a tree for support where three or more twigs spread from nearly the same place. Here the nuts, one in a place, were left till perhaps shaken to the ground by a severe wind or by some other cause. In one winter, without hunting for them, six to ten places were found in one neighborhood of Michigan, where something had placed a single walnut or acorn in the forks of small branches. In some cases a severe wind could have dislodged the nut.

On February 18, 1897, I found a single black walnut held by small branches of a red oak.

The oak was an inch and a half in diameter, and the nut was about six feet from the ground. The nearest bearing tree was fully three hundred long steps distant. We can imagine that, through fright or other causes, a squirrel might be suddenly interrupted while carrying nuts, and might then drop them to the ground, where later a tree would be started.

38. Birds scatter nuts. —The work of birds in scattering seeds and fruits has long been recognized.³

Some friends of mine collected a quantity of hazelnuts, while yet the green husks enclosed the nuts, and placed them near the house to dry. At once they were discovered by a blue jay, which picked out a nut at a time, flew away, held the nut between its toes, cracked it from the small end, and ate the contents. In this operation a number of nuts slipped away and were lost. But it seems that all were not eaten, for the next season half a dozen or more hazel shoots came up, and to-day a new patch of hazel bushes is growing in the yard. Doubtless many acorns are carried from place to place and dropped in an aimless way by woodpeckers, blue jays, and crows; also beechnuts by these birds, and by nuthatches, and by pigeons, before the latter became nearly extinct. Woodpeckers and blue jays place beechnuts and small acorns in the crevices of bark on standing trees. If left there very long, the nuts will become too dry to grow, but in the act of transporting them some of the nuts may be accidentally dropped in various places.

39. Do birds digest all they eat? —To determine whether seeds would lose their vitality in passing through the digestive organs of birds, Kerner von Marilaun fed seeds of two hundred and fifty different species of plants to each of the following: blackbird, song thrush, robin, jackdaw, raven, nutcracker, goldfinch, titmouse, bullfinch, crossbill, pigeon, fowl, turkey, duck, and a few others; also to marmot, horse, ox, and pig, making five hundred and twenty separate experiments. As to the marmot, horse, ox, and pig, almost all the fruits and seeds were destroyed. From the ox grew a very few seeds of millet, and from the horse one or two lentils and a few oats; from the pig a species of dogwood, privet, mallow, radish, and common locust. Under ordinary conditions, no seed was found to germinate after passing through the turkey, hen, pigeon, crossbill, bullfinch, goldfinch, nutcracker, titmouse, and the duck. Ravens and jackdaws passed without injury seeds of stone fruits and others with very hard coats. Of seeds that passed through the blackbird 75 per cent germinated, 85 per cent in the case of the thrush, 80 per cent in the case of the robin.⁴

40. Color, odor, and pleasant taste of fruits are advertisements. —In summer, buds are formed on bushes of black raspberry, blossoms appear, and these are followed by small, green, and bitter berries, which hardly anything cares to eat. They grow slowly, become soft and pulpy, and finally good to eat. How is bird or boy or girl to know where they are and when they are fit to eat? The plant has enterprise and has displayed two want advertisements by painting the berries first dark red, and then dark purple, when they are good to eat. But is the plant made expressly to produce berries, just to feed birds and children? If that be all, why are seeds formed in the berries in such large numbers? No! They produce berries that contain seeds, and from these seeds are to grow more bushes. Then why should not the berries always remain bitter or hard, so that nothing would touch them? If we may say so, the plant produces sweet and showy berries on purpose to be eaten, that the seeds may be carried away. What becomes of the seeds? Each one is enclosed in a hard, tough covering, which protects it from destruction in the stomachs of many birds and some other animals. The seeds are well distributed by the animals that eat the berries. The brilliant colors of ripe berries say to bird and child: "Here we are; eat us, for we are good." The sweet pulp pays the birds for distributing the seeds, else they would not be so distributed. The seeds are as well provided for locomotion as the ticks, the mites, and the spiders, and when ready to go, the berries flaunt their colors to attract attention. You see, then, that although the old parent bush cannot change its place, young bushes grow from the tips of the branches, and seedlings spring up at long distances from their old homes.

Sparrows, finches, and similar birds in the winter eat and destroy seeds of grasses and weeds, while the same birds in summer and autumn eat bushels of blueberries, huckleberries, elderberries, raspberries, strawberries, and similar fruits, and distribute their unharmed seeds over thousands of acres, which otherwise might never support a growth of these species.

The downy woodpecker, among other things, devours berries of three kinds of dogwood, Virginia creeper, service berry, strawberry, pokeberry, poison ivy, poison sumac, stag-horn sumac, and blue beech.

The hairy woodpecker devours many of the above fruits, as well as those of spicebush, sour gum, cherries, grapes, blackberries. The flicker devours most of the fruits listed for the two woodpeckers named above, also hackberry, black alder, green brier, bayberries. A number of other woodpeckers possess habits much the same as the three above named. The cedar bird devours many species of hard-seeded fruits.

The various shades of red appear to good advantage among green leaves. As illustrations of such, we have the wintergreen, partridge berry, bush cranberry, bearberry, service berry, currant, holly, strawberry, red-berried elder, winter berry, honeysuckle, and many more. Where the leaves are liable to become red in autumn the berries are often blue. Of such, notice wild grapes, blueberries, and berries of sassafras, though the flowering dogwood has red leaves as well as red berries.

There is a reason for prickles on rosebushes. When ripe, rosehips are usually red or yellow, and thus attract birds which are fond of the fleshy portion outside; but the seed-like nuts are too hard and dry to suit their taste, and are rejected and sown in the vicinity, where the ripened hips are picked in pieces and eaten. Mice and red squirrels are also fond of the seed-like nutlets of roses, but seldom secure them from the bushes. Why, do you ask? Because the prickles were most likely placed on the rosebushes to prevent this very thing, and not to annoy the lover of flowers, or to prevent her from cutting what she needs.

41. The meddlesome crow lends a hand. —"One of the most industrious and persistent seed-transporting agencies I know of is that ubiquitous, energetic, rollicking, meddlesome busybody, the crow. I have seen crows gather by hundreds and have a regular powwow, a mass convention, where they seemed to discuss measures and appoint officers. At length they get through, and as they start to fly away many, if not all, will drop something. I have found these to be acorns, walnuts, hickory nuts, buckeyes, sycamore balls, sticks, eggshells, pebbles, etc. As a crow leaves an oak he will pluck an acorn, which he may carry five miles and light on a beech tree where something else will attract his attention, when he will drop the acorn and maybe pluck a pod of beechnuts and fly away somewhere else." —Prof. W. B. Barrows.

The number of seeds distributed by crows is enormous, and consists of many species, including poison ivy and poison sumac, wild cherry, dogwood, red cedar, sour gum, and Virginia creeper. The hard, undigested seeds are mostly expelled from the mouth in pellets, shown in the illustration, and germinate more promptly than those untouched by birds.

Bears are very fond of berries, and will scatter the seeds of service berries, elder berries, chokecherries, raspberries, and blackberries.

42. Ants distribute some kinds of seeds. —Ants are numerous, strong, skillful, and in suitable weather are always very busy. Their habits have been investigated, and it has been found that in some respects they are genuine farmers on a small scale. They have their slaves (not hired help); they feed their plant lice, remove them from place to place, and otherwise care for them, because the lice constitute one of the chief sources of their supply of sweet. They build roads and houses, and enjoy society after their fashion. They have use for certain kinds of seeds, portions or all of which they eat at once or carry to their homes. A number of persons in different countries and at different times have seen ants carrying seeds. Some young student of botany may have noticed along one side of the glossy seeds of the bloodroot a delicate, fleshy ridge, and wondered what could be its use. The answer can now be given with a good degree of confidence. The ants either eat this fleshy ridge at once, or, as more frequently happens, carry such seeds to their homes. The smooth seeds they do not eat, but cast them out of their nests after using the part they like; after being rejected the seed may stand a chance to germinate. The seeds cannot be carried so well unless this ridge, caruncle, be present. Other seeds of this nature are those of wild ginger, celandine, cyclamen, violet, periwinkle, some euphorbias, bellwort, trillium, prickly poppy, dutchman's breeches, squirrel-corn, several species of Corydalis, Seneca snakeroot, and other species of milkworts.

In his work on Vegetable Mold and Earthworms, p. 113, Darwin states that earthworms are in the habit of lining their holes, using seeds among other things, and that these sometimes grow. In this way the worms aid in spreading plants.

43. Cattle carry away living plants and seeds. —In Arizona, where cacti abound, Professor Toumey finds that many of them are broken in pieces by cattle, which eat a portion, while other portions often adhere to the legs or noses and are carried from place to place. These fragments are usually capable of growing.

The unicorn plant, Martynia proboscidia, common in the southwestern portion of the United States, is sometimes seen in cultivation. When ripe, the fruit is hard, carrying two stout beaks with recurved tips. Experiments show it to be admirably adapted to catch on to the feet of sheep, goats, and cattle, or hold to the fleeces of the two former.

44. Water-fowl and muskrats carry seeds in mud. —Seeds and fruits of aquatic and bog plants that are floating, or in the mud of shallow water, are often carried by ducks, herons, swallows, muskrats, and other frequenters of such places, on their feet, beaks, or feathers, as they hastily leave one place for another. 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. Mud may be carried on the feet of land animals as well as on aquatic animals, not only from ponds and bogs, but from the fields where seeds may have accumulated in the earth or washed down the slopes.

45. Why some seeds are sticky. —Some seeds and fruits are sticky; in some instances the mucilaginous substance is normally moist enough to adhere to anything that touches it, while in other cases it requires to be wetted before it will adhere. The seeds of flax, plantain, peppergrass, basil, sage, dracocephalum, groundsel, drop-seed grass, and many others less familiar, possess this peculiarity. The berries of some plants, when fully ripe, burst very easily when touched, and some of the seeds are then likely to adhere to animals and be carried away. Some berries of several plants belonging to the nightshade family have this peculiarity, as well as some of the cucurbits. When the outer covering of seeds of water lilies, arums, and others are broken, the gummy secretion is very likely to adhere to the feathers, or fur, or feet of animals. A number of fruits, and even the upper fruit-bearing branches, have sticky glands with which to catch on to any passing object. Among these are some kinds of sedges, chickweeds, and catchflies.

The sticky substance on seeds and fruits not unfrequently serves another good turn besides enabling them to adhere to animals. The slime holds them to the spot where they are to grow, or it enables some to float or to sink in water, according to the amount of the mucilage.

46. Three devices of Virginia knotweed. —A perennial plant, four to five feet high, grows on low land, usually in the shade. It is Polygonum Virginicum, and so far without a common name, unless Virginia knotweed be satisfactory. It is a near relative of knot grass and smartweed and Prince's feather. 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. Observe the slender, stiff beak, terminating in two recurved points. Let a person or some animal pass into a patch of these plants, and at once numerous fruits catch on wherever there is a chance, and some are shot upon or into the fleeces of animals, there to find free transportation for uncertain distances. Should there be a freshet, some of these fruits will float; or, in case of shallow currents after a rain, some of them are washed away from the parent plant. Any inquisitive person cannot fail to be pleased if he experiment with the plant when the fruit is ripe.

47. Hooks rendered harmless till time of need. —There are a number of rather weedy-looking herbs, common to woods or low land, known as Avens, Geum. They are closely allied to cinquefoil, and all belong to the rose family. The slender stiles above the seed-like ovaries of some species of Avens are described as not jointed, but straight and feathery, well adapted, as we might suppose, to be scattered by the aid of wind; while others are spoken of as having, when young, stiles jointed and bent near the middle. In ripening, the lower part of the stile becomes much longer and stouter. When a whole bunch of pistils has drawn all the nourishment possible, or all that is needed, from the plant mother, the upper part of each stile drops off, leaving a sharp, stiff hook at the end. At this time each pistil loosens from the torus and can be easily removed, especially if some animal touch the hooks. To help in holding fast to animals, there are a number of slender hairs farther down the stile, which are liable to become more or less entangled in the animal's hair, fur, wool, or feathers. Even in the small number of plants here noticed, we have seen that scarcely any two of them agree in the details of their devices for securing transportation of seeds. I know of nothing else like the Geum we are now considering. When young and green, the tip of each hook is securely protected by a knob or bunch, with a little arm extending above, which effectually prevents the hook from catching on to anything; but, when the fruit is ripe, the projecting knob with its little attachment disappears. The figures make further description unnecessary. To keep the plow from cutting into the ground while going to or from the field, the farmer often places a wooden block, or "shoe," over the point and below the plow. Sometimes we have known persons to place knobs of brass or wood on the tips of the sharp horns of some of their most active or vicious cattle, to prevent them from hooking their associates or the persons having them in charge. Nature furnishes the points of the young fruits of some species of Avens with knobs, or shoes, for another purpose, to benefit the plants without reference to the likes or dislikes of animals.