Popular Science Monthly/Volume 43/June 1893/Adaptations of Seeds and Fruits
|ADAPTATIONS OF SEEDS AND FRUITS.|
IF we consider the great variety of seeds and fruits, we naturally inquire its meaning; and if we are sufficiently interested to observe carefully the part which seeds play in Nature, we soon find that in innumerable ways they are adapted to their surroundings. On the seed, primarily, rests the all-important responsibility of perpetuating the species, and success or failure in this duty depends upon the manner in which the seed is adapted to encounter the dangers that threaten it.
The manifold adaptations of this kind which Nature exhibits have been brought about chiefly by natural selection, resulting from the cooperation of two laws: the law of heredity and the law of variation. Under the former, characteristics of a parent are transmitted to its offspring. In obedience to the latter, no offspring is exactly like its parent, but differs from it more or less. The variation being inherited by the succeeding generation will, if of favorable nature, tend to be perpetuated indefinitely. Contrarily, variation in an unfavorable direction will conduce to extermination of the species from the very nature of the case. Thus it follows that the accumulation of advantageous variations, however slight, and the necessary destruction of species possessing unfriendly characteristics, results in producing kinds well fitted for existence.
Bearing the above in mind as a general explanation, let us consider some of its effects as displayed in seeds and fruits.
We usually find seeds in a seed vessel of some sort, the whole affair constituting the "fruit." Common to all immature fruits is their necessity for protection, and this is met in various ways. Winds which would break them off are effectually resisted by their strong yet flexible footstalks; and possible injury by bruising is averted by tough, elastic walls, often cushioned by prickles or other appendages.
Sudden changes of temperature, before they can penetrate to the unripe seeds, are rendered harmless by the blanketing effect of pulp or other material.
For protection from the animal world, immature fruits have developed a number of interesting devices. Almost universally "green" fruits so harmonize with surrounding color as readily to escape detection. In fact, the hazelnut is enveloped in a leafy coat which renders it very inconspicuous. The nutritious albumen of the seed is often fortified by such impenetrable shells as those of the cocoanut and others. Perhaps there is a formidable armament of prickles, as in the chestnut; or of stinging hairs, as is the case with some pods. Characteristic of immature fruits are disagreeable taste and consistence. Compare an unripe peach, sour and stringy, with the same fruit in its luscious maturity.
But all these contrivances fail to repel certain enemies of growing fruits. The apple's inconspicuousness, toughness, and sourness are of little avail against the young progeny of the genus Homo.
In many remarkable instances plants by their movements are able to protect their precious seeds from injury. In our common fall dandelion the whole flower closes up while the seeds are ripening, but reopens at their maturity. Furthermore, the upright flower stalk sinks to the ground when the flowers fade, but erects itself again when the seeds are ready to be scattered by the wind. In one of our winter house plants, the common cyclamen, the flower stalk coils up after flowering, bringing the pod to the ground to ripen; and our sweet white water lily, after expanding and withering above water, sinks to mature its seeds in safety. Other more remarkable but less common cases might be cited to show the extreme care with which plants preserve their seeds from possible destruction.
At maturity the one object of the seed is to secure the advantages of wide dispersion, and to effect this purpose Nature uses all means at hand. The agencies against which she so lately contrived are now most sedulously sought, and almost endless are the modifications of structure which enable seeds to spread far and wide.
"Dehiscence," the splitting open of a ripe pod, is manifestly a provision for seed dispersion. In its simplest form dehiscence merely exposes seeds to various conveying agencies: to the wind, in the milkweed; to birds, in the case of some brightly colored beans. Other plants, however, do more than this. Our wild columbine is particularly well adapted for having its seeds scattered by the wind. They are held in open seed vessels surmounting a slender stalk which, although nodding at flowering time, has become upright. A slight breeze easily shakes this stalk, causing the seeds to be thrown for quite a distance. The poppy throws its seeds in a similar way, and the little eaves which stand over the holes in the pod are even said to close in wet weather, not allowing the seeds to escape.
In many wonderful instances the ripe pod projects its seeds forcibly into the air. In some of our wild violets the pod, after dehiscence, consists of three spreading valves, each shaped like a boat, bearing within several seeds which are pear-shaped, hard, and smooth. In drying, the valve walls contract, approach each other, and squeeze out the seeds, which are thus thrown several feet. Our wild witch-hazel throws its seeds often to the distance of thirty feet. Many of us recollect the sudden bursting and coiling up of the pods of the "touch-me-not," whose yellowish, spurred flowers are so common in moist places. The object of this action is to expel the seeds. Curious is the case of the squirting cucumber of southern Europe. The ripe, cucumber-like fruit is greatly distended by its contents. At a slight touch, as from a browsing animal, it breaks from the stalk, and through the hole thus formed the pressure of the elastic walls forces the seeds in a viscid liquid for twenty or thirty feet.
Fruits that do not split open are invariably scattered by external means, inanimate and animate. Of inanimate agencies the wind is far oftenest employed, and seeds have evidently found it extremely efficient, judging from their many adaptations for wind dispersion. The seeds of our elms, maples, pines, etc., are surrounded, as we know, by thin expansions called "wings," whose purpose plainly is to present a large surface for the wind to act upon. Wings are characteristic alone of trees or tall shrubs, and never occur on low herbs, where they would clearly be out of place. Instead of a wing, a tuft of hairs frequently serves the same purpose. A common example is furnished by the milkweed, whose seed is surrounded by a spreading "pappus" of long, silky hairs. The dandelions and thistles have adopted this means of distribution, and this explains their abundance everywhere. In the smoke-bush of our gardens only a few flower stalks bear fruit, the rest become slender and feathery, forming a light network which is borne along in the wind, carrying the few small fruits which have formed.
Flowing water transports many large nuts, some depending upon it almost exclusively. Drifting along in our fresh-water streams one may often see the "key fruits" of the red maple, and the soaking they thus receive must further germination. The prevalence on our river banks of oaks, hickories, and maples is also very noticeable. Again, ocean currents are of great importance in distributing plants. The cocoanut, buoyed by its loose husk and protected by an impenetrable shell, floats in the sea until it is brought often to some coral island where it may grow. Many small seeds are also conveyed by ocean currents, and it is very probable that they retain their vitality, for Mr. Darwin has recorded some interesting experiments showing that a good proportion of seeds can withstand injury from salt water for a considerable length of time. The action of freezing water, as manifested in frost, has the well-known effect of freeing nuts from their protecting envelopes; and frozen water, in the shape of glaciers and icebergs, is of a little importance in transporting seeds. It is possible that during the Glacial period seeds were conveyed from place to place incased in ice.
Of all devices for dispersion the most remarkable are those by which the aid of animals is secured, and this aid is so valuable that plants spare no expense to obtain it. Usually animals are well paid for their services, but many plants, however, do not hesitate to deceive their benefactors by all sorts of trickery. This latter class, though, has not been nearly as successful as the others in the struggle for existence.
It is now well known that what are popularly called "fruits" exist for the mutual benefit of plants and the lower animals—not for man. And it is generally believed that these fruits have developed their attractive qualities through natural selection. The results reached by man in selecting and propagating the best varieties of fruits are the strongest grounds for thinking that these fruits were once evolved from very crude conditions through similar selection by the lower animals, particularly birds. Such fruits, for instance, as by natural variation became at all agreeable to birds would be sought out by them, to the exclusion of less attractive fruits. In consequence, the favored fruits would stand better chances of setting seeds than would their less favored companions. Variations being transmitted from parent to off-spring, it is reasonable to suppose that favorable variations would become still more favorable by further selection, until, by the accumulation of even slight variations through geologic ages, there would result fruits highly attractive to certain animals by their color, perfume, and taste. In the mean time, fruits possessing unfavorable characteristics have for this very reason been exterminated, or else have attained a less degree of success than the others.
Insects are the lowest animals known to assist in seed dissemination. Mr. Darwin tells us of locust excrement containing seeds which grew when planted. Considering that locusts often occur in vast swarms, they can hardly fail to be highly effective agents in seed dissemination, thus repaying to some extent for the immense damage they often do.
Fishes are known to swallow seeds of many kinds, and must transport them from place to place; but the value of fishes as seed conveyers is hard to estimate.
We have just said that our edible fruits are really contrivances for securing seed dissemination, especially through the agency of birds. Take, for example, some of our common fruits—the currant, grape, plum, peach, apple, etc. All these are constructed with this end in view. When ripe, they are colored brightly to attract animals; some possess agreeable odors, and most have a delicious taste and consistence. In short, they are highly adapted to become the food of animals. While swallowing such food animals can hardly help swallowing seeds as well, and such seeds are finally emitted under conditions admirably conducive to germination. Why our most delicious fruits are often offset by their disagreeable seeds may have occurred to many of us. The fact is, by this means seeds are protected from possible injury in the alimentary canals of animals. Take, for example, the small, hard seeds of the grape or fig, and the similar so-called seeds of the strawberry, blackberry, and others. Far from being destroyed by the digestive juices, the seeds are probably facilitated in their germination by the warmth and moisture received.
The rapid ripening of fruits doubtless prevents their premature destruction. The accompanying change in color is remarkable. Whereas young fruits harmonize completely with surrounding color, mature fruits are extremely conspicuous. Recall the barberry, rose, sumach, mountain ash, and many more. In some honeysuckles each cluster of scarlet berries stands in violent contrast against a green leaf. In the blackberry lily of our gardens the sides of the pod roll back and display their white linings, conspicuously relieving the black, berry-like seeds. The burning-bush is a brilliant example with its flaming scarlet. In the West Indies is a plant whose pods are red within, containing seeds that are blue. Other instances might be named, but they are indefinitely numerous and easily observed by any one.
Many of our fruits are covered with a waxy "bloom" as it is called. This is plainly a protection, for it is commonly known that fruits will long resist decay provided this coating is uninjured. Its probable effect is to resist decomposition by moisture and fungi.
The edible portion, however, is of most interest to us, not only scientifically, but also in a practical way. How highly it is esteemed by some animals may be judged from the expense we often incur in buying fruits out of season.
The use of poisonous fruits is an interesting subject for consideration. How is a plant benefited by producing them?
Mr. Grant Allen suggests with regard to a near relative of our Jack-in-the-pulpit that its brilliant scarlet berries are readily detected and eaten by birds; that such birds are consequently poisoned, and by decaying provide abundant nourishment for the germinating seeds. He adds that birds can not profit by experience and avoid the berries, as no bird ever lives to tell the tale.
At first this explanation seems very reasonable, and perhaps it is; but we have reason for doubting it, for we find that many fruits poisonous to mammals are eaten by birds without the slightest injury. The beautiful apple-like manchineel, which is most virulently poisonous, is eaten by tropical birds with the greatest impunity.
On the whole it seems very likely that some fruits are fatal to other animals but not to birds, and under all explanations poisons are doubtless a protection, at least, to the fruits which possess them.
Many fruits have been so highly cultivated by man that they can no longer set their seeds as originally. Our wild red cherry is a convenient morsel for even small birds; but its highly cultured relatives of the garden must submit their flesh to birds who can not eat stones as well. The case of the strawberry is different, however, for birds can scarcely take a morsel that does not contain numbers of the small, hard "straws," which are really the most essential parts of the plant, for each one incloses a seed.
In many cases Nature economically develops as little sweet pulp as will serve her purpose. In the wild red cherry, for instance, the stone occupies almost the entire fruit, there being only a thin layer of food substance. Often there is none whatever, and instead the fruit attains its ends by simulated attractiveness. The rosary bean temptingly displays its brilliant red seeds, which are in reality of stony hardness. Yet it does not wholly rely upon this artifice, for it is very probable that part of the seeds are scattered by the twisting dehiscence of the tough pod.
In some instances the deception is really wonderful. Some pods and seeds mimic insects so closely as probably to entice insectivorous birds to carry them, at least until the birds find out their mistake. It may be also that this appearance protects them from graminivorous birds. There are pods which curiously resemble worms and spiders and caterpillars. Our common castor-oil bean bears a superficial likeness to a beetle. Yet there are some most remarkable cases of mimicry where beetles are counterfeited in the minutest detail.
Fruits are also disseminated by mammals as well as birds. Berries are the favorite food of many of our native mammals, the woodchuck and others. Wild apples are frequently carried off by squirrels, and it is well known that squirrels store up large quantities of nuts which oftentimes are never eaten. Fruits too large to be swallowed by most birds are easily devoured by the larger mammals, the apple, for instance, whose seeds are protected by tight husks well adapted to slip through the alimentary canal of an animal without receiving the least injury.
The gourd fruits, so much liked by man here, are equally attractive to his quadrumanal brothers in the tropics.
For utilizing the services particularly of mammals many fruits have developed hooks or horns to catch in the fleece of passing creatures, who thus transport the seeds from place to place. An autumn tramp through our pastures will soon convince one of the efficiency of this mode of dissemination.
A very familiar example of this kind we find in the common burdock; but the hooks of the burdock are insignificant affairs compared with some which exist. In the Southern States grows a fruit, Martynia proboscidea, having two recurving horns several inches long. The appearance of the fruit would justify its having an even more formidable name. Another fruit, Harpagophyton by name, is a bristling mass of powerful hooks. It is said that lions trying to free themselves from its clutches get it into the mouth and die in torture. Instead of hooks, seeds sometimes effect the same purpose by being sticky.
It is a suggestive fact that hooked fruits occur on low plants, never on trees; also that in geologic time hooks appeared simultaneously with land mammals.
Lastly, we must recollect that man himself disseminates seeds in a thousand ways. War often introduces new plants into a region. Commerce is of vast importance in this respect. In the vicinity of our woolen mills a strange flora, from seeds introduced with the raw wool, is struggling with native plants. Agriculture is certainly of unbounded effect in the way we are considering. In short, human will has almost limitless control over the circumstances of plant life.
After dispersion most seeds simply rest on the ground to await germination, perhaps protected by color resemblance, as in nuts, or by mimicry, sometimes mimicking a dry twig to perfection. Some seeds, though, do more than this. The parasitic seeds of the mistletoe, dropped by birds on the boughs of trees, would soon fall to the ground and die were they were not very sticky. The seed of Mysodendron has three long, flexible appendages which twine round any suitable branch to which it is blown.
There are a few seeds which literally corkscrew themselves into the ground. One of our natives—Erodium, or cranesbill—has seeds which are small, pointed, and covered with hairs. The posterior end is prolonged into a hairy, corkscrew-like awn, which twists or untwists, according to the amount of moisture. This awn ends in a feather-like affair with backward-pointed hairs. On moist ground the seed-hairs stand out so as to place the seed-point downward, and the awn untwists; but the barbed feather preventing upward movement, because it catches in the herbage, the seed is forced into the soil. However, if the awn dries and contracts, the feather is easily drawn down while the seed is not drawn up. By successive moistenings and dryings the seed is ultimately driven completely into the earth.
As to vitality, seeds present widest differences. Very short-lived seeds are those of the coffee and magnolia. On the other hand, under abnormal conditions, some seeds have retained vitality for many centuries, apparently. Raspberry seeds, found in a Celtic tumulus along with coins of the Emperor Hadrian, germinated, according to good authority, after a possible interval of several centuries. Other seeds from old Roman tombs grew after a lapse of many hundred years, but these are exceptional instances. Accurate experiments show that a few kinds live for fifteen years, or thereabouts, while the majority are much shorter lived. Stories of wheat raised from seed found in mummy wrappings are founded upon no trustworthy evidence whatever.
When a forest has been removed by fire, or otherwise, it commonly happens that a fresh growth of entirely new plants immediately springs up. This may be partly due to the unusual opportunity for growth thus given to foreign seeds; but the usually accepted explanation is that the new growth is from seeds which have long lain dormant.
Finally, as regards germination, seeds accommodate themselves to surrounding conditions with considerable readiness. Some seeds are so tenacious of life as to germinate, not only when old, but also when a large share of their food substance has been destroyed, provided, of course, that the germ itself is uninjured. No seed, however, will germinate without the proper amount of moisture, free oxygen, and warmth, although other disadvantages are often withstood successfully.
We have now described some of the more evident adaptations to surroundings displayed in seeds and fruits, but by no means all; for here, as everywhere else, Nature presents a variety which is almost infinite. Although endless differences in structure are still unexplained, we must believe that they are adaptations to circumstances present or past, and our knowledge leads us confidently to expect that future discovery will reveal in increased vastness the complexity of the relations by which everything in Nature is adapted, more or less perfectly, to everything else.