Popular Science Monthly/Volume 19/July 1881/On Fruits and Seeds II
|ON FRUITS AND SEEDS.|
By Sir JOHN LUBBOCK, F. R. S.
IN a very large number of cases the diffusion of seeds is effected by animals. To this class belong the fruits and berries. In them an outer fleshy portion becomes pulpy, and generally sweet, inclosing the seeds. It is remarkable that such fruits, in order, doubtless, to attract animals, are, like flowers, brightly colored—as, for instance, the cherry, currant, apple, peach, plum, strawberry, raspberry, and many others. This color, moreover, is not present in the unripe fruit, but is rapidly developed at maturity. In such cases the actual seed is generally protected by a dense, sometimes almost stony, covering, so that it escapes digestion, while its germination is perhaps hastened by the heat of the animal's body. It may be said that the skin of apple and pear pips is comparatively soft; but then they are imbedded in a stringy core, which is seldom eaten.
These colored fruits form a considerable part of the food of monkeys in the tropical regions of the earth, and we can, I think, hardly doubt that these animals are guided by the colors, just as we are, in selecting the ripe fruit. This has a curious bearing on an interesting question as to the power of distinguishing color possessed by our ancestors in bygone times. Magnus and Geiger, relying on the well known fact that the ancient languages are poor in words for color, and that in the oldest books—as, for instance, in the Vedas, the Zend-Avesta, the Old Testament, and the writings of Homer and Hesiod—though, of course, the heavens are referred to over and over again, its blue color is never dwelt on, have argued that the ancients were very deficient in the power of distinguishing colors, and especially blue. In our own country Mr. Gladstone has lent the weight of his great authority to the same conclusion. For my part I can not accept this view. There are, it seems to me, very strong reasons against it, into which I can not, of course, now enter; and, though I should rely mainly on other considerations, the colors of fruits are not, I think, without significance. If monkeys and apes could distinguish them, surely we may infer that even the most savage of men could do so too. Zeuxis would never have deceived the birds if he had not had a fair perception of color.
Fig. 14.—a, burdock (Lappa); b, agrimony (agrinmonia); c, bur parsley (Caucalis); d, enchanter's nightshade (Circœa); e, cleavers (Galium); f, forget-me-nots (Myosotis).
In these instances of colored fruits, the fleshy edible part more or less surrounds the true seeds; in others the actual seeds themselves become edible. In the former the edible part serves as a temptation to animals; in the latter it is stored up for the use of the plant itself. When, therefore, the seeds themselves are edible they are generally protected by more or less hard or bitter envelopes, for instance the horse-chestnut, beech, Spanish chestnut, walnut, etc. That these seeds are used as food by squirrels and other animals is, however, by no means necessarily an evil to the plant, for the result is that they are often carried some distance and then dropped, or stored up and forgotten, so that in this way they get carried away from the parent tree.
In another class of instances animals, unconsciously or unwillingly, serve in the dispersion of seeds. These cases may be divided into two classes, those in which the fruits are provided with hooks, and those in which they are sticky. To the first class belong, among our common English plants, the burdock (Lappa, Fig. 14 a), agrimony (Agrimonia, Fig. 14 b); the bur parsley (Caucalis, Fig. 14 c); enchanter's nightshade (Cicræa, Fig. 14 d); goose-grass or cleavers (Galium, Fig. 14 e), and some of the forget-me-nots (Myosotis, Fig. 14 f). The hooks, moreover, are so arranged as to promote the removal of the fruits. In all these species the hooks, though beautifully formed, are small; but in some foreign species they become truly formidable. Two of the most remarkable are represented on page 357—Martynia proboscidea (Fig. 15 b) and Harpagophyton procumbens (Fig. 15 a). Martynia is a plant of Louisiana, and if its fruits once get hold of an animal it is most difficult to remove them. Harpagophytum is a South African genus. The fruits are most formidable, and are said sometimes even to kill lions. They roll about over the dry plains, and, if they attach themselves to the skin, the wretched animal tries to tear them out, and sometimes getting them into his mouth perishes miserably.
The cases in which the diffusion of fruits and seeds is affected by their being sticky are less numerous, and we have no well-marked instance among our native plants. The common plumbago of South Europe is a case which many of you no doubt have observed. Other genera with the same mode of dispersion are Pittosporum, Pisonia, Boerhavia Slegesbeckia, Grindelia, Drymaria, etc. There are comparatively few cases in which the same plant uses more than one of these modes of promoting the dispersion of its seeds, still there are some such instances. Thus in the common burdock the seeds have a pappus, while the whole flower-head is provided with hooks which readily attach themselves to any passing animal. Asterothrix, as Hildebrand has pointed out, has three provisions for dispersion; it has a hollow appendage, a pappus, and a rough surface.
But perhaps it will be said that I have picked out special cases; that others could have been selected, which would not bear out, or perhaps would even negative, the inferences which have been indicated; that I have put the cart before the horse; that the ash-fruit has not a wing in order that it may be carried by the wind, or the burdock hooks that the heads may be transported by animals, but that, happening to have wings and hooks, these seeds are thus transported. Now, doubtless there are many points connected with seeds which are still unexplained; in fact, it is because this is so that I was anxious to direct attention to the subject. Still I believe the general explanations which have been given by botanists will stand any test.
Let us take, for instance, seeds formed on the same type as that of the ash—heavy fruits, with a long wing, known to botanists as a samara.
Fig. 15.—a. Harpagophyton procumbens (natural size); b, Martynia proboscidea (natural size).
Now, such a fruit would be of little use to low herbs, which, however, are so numerous. If the wing was accidental, if it was not developed to serve as a means of dispersion, it would be as likely to occur on low plants and shrubs as on trees. Let us, then, consider on what kind of plants these fruits are found. They occur on the ash, maple, sycamore, hornbeam, pines, firs, and elm; while the lime, as we have seen, has also a leaf attached to the fruits, which answers the same purposes. Seeds of this character therefore occur on a large proportion of our forest-trees, and on them alone. But more than this: I have taken one or two of the most accessible works in which seeds are figured, for instance, Gärtner's "De Fructibus et Seminibus," Le Maout and Decaisne's (Hooker's translation) "Descriptive and Analytical Botany," and Baillon's "Histoire des Plantes." I find thirty genera, belonging to twenty-one different natural orders, figured as having seeds or fruits of this form. They are all trees or climbing shrubs, not one being a low herb.
Let us take another case, that of the plants in which the dispersion of the seeds is effected by means of hooks. Now, if the presence of these hooks was, so to say, accidental, and the dispersion merely a result, we should naturally expect to find some species with hooks in all classes of plants. They would occur, for instance, among trees and on water-plants. On the other hand, if they are developed that they might adhere to the skin of quadrupeds, then, having reference to the habits and size of our British mammals, it would be no advantage for a tree or for a water-plant to bear hooked seeds. Now, what are the facts? There are about thirty English species in which the dispersion of the seeds is effected by means of hooks, but not one of these is aquatic, nor is one of them more than four feet high. Nay, I might carry the thing further. We have a number of minute plants, which lie below the level at which seeds would be likely to be entangled in fur. Now, none of these, again, have hooked seeds or fruits. It would also seem, as Hildebrand has suggested, that in point of time, also, the appearance of the families of plants in which the fruits or seeds are provided with hooks coincided with that of the land mammalia.
Again, let us look at it from another point of view. Let us take our common forest-trees, shrubs, and tall, climbing—plants not, of course, a natural or botanical group, for they belong to a number of different orders, but a group characterized by attaining to a height of say over eight feet. We will in some cases only count genera; that is to say, we will count all the willows, for instance, as one. These trees and shrubs are plants with which you are all familiar, and are about thirty-three in number. Now, of these thirty-three no less than eighteen have edible fruits or seeds, such as the plum, apple, arbutus, holly, hazel, beech, and rose. Three have seeds which are provided with feathery hairs; and all the rest, namely, the lime, maple, ash, sycamore, elm, hop, birch, hornbeam, pine, and fir, are provided with a wing. Moreover, as will be seen by the following table, the lower trees and shrubs, such as the cornel, Guelder rose, rose, thorn, privet, elder, yew, and holly, have generally edible berries, much eaten by birds. The winged seeds or fruits characterize the great forest trees.
TREES, SHRUBS, AND CLIMBING SHRUBS NATIVE OR NATURALIZED IN BRITAIN.
|Seed or Fruit.|
|Lime (Tilia Europæa)||X|
|Guelder rose (Viburnum)||X|
Or let us take one natural order. That of the roses is particularly interesting. In the genus Geum the fruit is provided with hooks; in Dryas it terminates in a long feathered awn, like that of Clematis. On the other hand, several genera have edible fruits; but it is curious that the part of a plant which becomes fleshy, and thus tempting to animals, differs considerably in the different genera. In the blackberry, for instance, and in the raspberry, the carpels constitute the edible portion. When we eat a raspberry we strip them off and leave the receptacle behind; while in the strawberry the receptacle constitutes the edible portion; the carpels are small, hard, and closely surround the seeds. In these genera the sepals are situated below the fruit. In the rose, on the contrary, it is the peduncle that is swollen and inverted, so as to form a hollow cup, in the interior of which the carpels are situated. Here you will remember that the sepals are situated above, not below, the fruit. Again, in the pear and apple, it is
the ovary which constitutes the edible part of the fruit, and in which the pips are imbedded. At first sight, the fruit of the mulberry—which, however, belongs to a different family—closely resembles that of the blackberry. In the mulberry, however, it is the sepals which becomes fleshy and sweet.
The next point is that seeds should be in a spot suitable for their growth. In most cases the seed lies on the ground, into which it then pushes its little rootlet. In plants, however, which live on trees, the case is not so simple, and we meet some curious contrivances. Thus, the mistletoe, as we all know, is parasitic on trees. The fruits are eaten by birds, and the droppings often, therefore, fall on the boughs; but if the seed were like that of most other plants it would soon fall to the ground and consequently perish. Almost alone among English plants it is extremely sticky and thus adheres to the bark.
Fig. 16.—Myzodendron. (After Hooker.)
I have already alluded to an allied genus, Arceuthobium, parasitic on junipers, which throws its seeds to a distance of several feet. These also are very viscid, or, to speak more correctly, are imbedded in a very viscid mucilage, so that if they come in contact with the bark of a neighboring tree they stick to it.
Another interesting genus, again of the same family, is Myzodendron (Fig. 16), a Fuegian species, described by Sir Joseph Hooker, and parasitic on the beech. Here the seed is not sticky, but is provided with four flattened, flexible appendages. These catch the wind, and thus carry the seed from one tree to another. As soon, however, as they touch any little bough, the arms twist round it and there anchor the seed.
In many epiphytes the seeds are extremely numerous and minute. Their great numbers increase the chance that the wind may waft some of them to the trees on which they grow; and as they are then fully supplied with nourishment they do not require to carry any store with them. Moreover, their minute size is an advantage, as they are carried into any little chink or cranny in the bark, while a larger or heavier seed, even if borne against a suitable tree, would be more
Fig. 17.—Cardamine chenopodifolia. a a, ordinary pods; b, subterranean pods.
likely to drop off. In the genus Neumannia, the small seed is produced at each end into a long filament which must materially increase its chances of adhering to a suitable tree
Even among terrestrial species there are not a few cases in which plants are not contented simply to leave their seeds on the surface of the soil, but actually sow them in the ground.
Thus in Trifolium subterraneum, one of our rarer English clovers, only a few of the florets become perfect flowers, the others form a rigid, pointed head, which at first is turned upward, and, as their ends are close together, constitute a sort of spike. At first, I say, the flower-heads point upward like those of other clovers, but, as soon as the florets are fertilized, the flower-stalks bend over and grow downward, forcing the flower-head into the ground, an operation much facilitated by the peculiar construction and arrangement of the imperfect florets. The florets are, as Darwin has shown, no mere passive instruments. So soon as the flower-head is in the ground they begin, commencing from the outside, to bend themselves toward the peduncle, the result of which, of course, is to drag the flower-head farther and farther into the ground. In most clovers each floret produces a little pod. This
Fig. 18.—Vicia amphicarpa. a a, ordinary pods; b b, subterranean pods.
would in the present species be useless, or even injurious; many young plants growing in one place would jostle and starve one another. Hence we see another obvious advantage in the fact that only a few florets perfect their seeds.
I have already alluded to our cardamines, the pods of which open elastically and throw their seed some distance. A Brazilian species, C. chenopodifolia (Fig. 17), besides the usual long pods (Fig. 17, a a), produces also short, pointed ones (Fig. 17, b b), which it buries in the ground.
Arachis hypogœa is the ground-nut of the West Indies. The flower is yellow and resembles that of a pea, but has an elongated calyx, at the base of which, close to the stem, is the ovary. After the flower has faded, the young pod, which is oval, pointed, and very minute, is carried forward by the growth of the stalk, which becomes two or three inches long and curves downward, so as generally to force the pod into the ground. If it fails in this, the pod does not develop, but soon perishes; on the other hand, as soon as it is underground, the pod begins to grow and develops two large seeds.
In Vicia amphicarpa (Fig. 18), a south European species of vetch.
Fig. 19.—Lathtrus amphicarpos. (After Sowerby.) a, ordinary pods; b, subterranean pods.
there are two kinds of pods. One of the ordinary form and habit (a), the other (b) oval, pale, containing only two seeds, borne on underground stems, and produced by flowers which have no corolla.
Again, a species of the allied genus Lathyrus (Fig. 19), L. amphicarpos affords us another case of the same phenomenon.
Other species possessing the same faculty of burying their seeds are Okenla hypogæa several species of Commelyna, and of Amphicarpæa, Voandzeia subterranea, Scrophularia arguta, etc.; and it is very remarkable that these species are by no means nearly related, but belong to distinct families, namely, the Cruciferœ, Leguminosœ, Commelynaceæ Violaceæ, and Scrophulariaceæ.
Moreover, it is interesting that in L. amphicarpos as in Vicia amphicarpa and Cardamine chenopodifolium, the subterranean pods differ from the usual and aërial form in being shorter and containing fewer seeds. The reason of this is, I think, obvious. In the ordinary pods the number of seeds of course increases the chance that some will find a suitable place. On the other hand, the subterranean ones are carefully sown, as it were, by the plant itself. Several seeds together would only jostle one another, and it is therefore better that one or two only should be produced.
In the Erodiums, or cranesbills, the fruit is a capsule which open elastically, in some species throwing the seeds to some little distance. The seeds themselves are more or less spindle-shaped, hairy, and produced into a twisted hairy awn as shown in Fig. 20, representing a seed of E. glaucophyllum. The number of spiral turns in the awn depends upon the amount of moisture; and the seed may thus be made into a very delicate hygrometer, for, if it be fixed in an upright position, the awn twists or un-twists according to the degree of moisture, and its extremity thus may be so arranged as to move up) and down like a needle on a register. It is also affected by heat. Now, if the awn were fixed instead of the seed, it is obvious that, during the process of untwisting, the seed itself would be pressed downward, and, as M. Roux has shown, this mechanism thus serves actually to bury the seed. His observations were made on an allied species, Erodium ciconium, which he chose on account of its size. He found that, if a seed of this plant is laid on the ground, it remains quiet as long as it is dry; but as soon as it is moistened—i. e., as soon as the earth becomes in a condition to permit growth—the outer side of the awn contracts, and the hairs surrounding the seed commence to move outward, the result of which is gradually to raise the seed into an upright position with its point on the soil. The awn then commences to unroll, and consequently to elongate itself upward, and it is obvious that, as it is covered with reversed hairs, it will probably press against some blade of grass or other obstacle, which will prevent its moving up, and will therefore tend to drive the seed into the ground. If, then, the air becomes drier, the awn will again roll up, in which action M. Roux thought it would tend to draw up the seed, but from the position of the hairs the feathery awn can easily slip downward, and would therefore not affect
Fig. 21.—Seed of Stipa pennata. (Natural size.)
the seed. When moistened once more, it would again force the seed farther downward, and so on until the proper depth was obtained. A species of anemone (A. montana) again, has essentially the same arrangement, though belonging to a widely separated order.
A still more remarkable instance is afforded by a beautiful South European grass, Stipa pennata (Fig. 21), the structure of which has been described by Vaucher, and more recently, as well as more completely, by Frank Darwin. The actual seed is small, with a sharp point, and stiff, short hairs pointing backward. The posterior end of the seed is produced into a fine twisted corkscrew-like rod, which is followed by a plain cylindrical portion, attached at an angle to the corkscrew, and ending in a long and beautiful feather, the whole being more than a foot in length. The long feather, no doubt, facilitates the dispersion of the seeds by wind; eventually, however, they sink to the ground, which they tend to reach; the seeds being the heaviest portion, point downward. So the seed remains as long as it is dry, but if a shower comes on, or when the dew falls, the spiral unwinds, and if, as is most probable, the surrounding herbage or any other obstacle prevents the feathers from rising, the seed itself is forced down and so driven by degrees into the ground.
I have already mentioned several cases in which plants produce two kinds of seeds, or at least of pods, the one being adapted to burying itself in the ground. Heterocarpism, if I may term it so, or the power of producing two kinds of reproductive bodies, is not confined to these species. There is, for instance, a North African species of corydalis (C. heterocarpa of Durieu) which produces two kinds of seed (Fig. 22), one somewhat flattened, short, and broad, with rounded angles; the other elongated, hooked, and shaped like a shepherd's crook with a thickened staff. In this case the hook in the latter form perhaps serves for dispersion.
Our common Thrincia hirta (Fig. 13, b) also possesses, besides the fruits with the well-known feathery crown, others which are destitute of such a provision, and which probably, therefore, are intended to take root at home. Mr. Drummond, in the volume of "Hooker's Journal of Botany" for 1842, has described a species of Alisimaceæ which has two sorts of seed-vessels; the one produced from large, floating flowers, the other at the end of short, submerged stalks. He does not, however, describe either the seeds or seed-vessels in detail.
Before concluding, I will say a few words as to the very curious forms presented by certain seeds and fruits. The pods of Lotus, for instance, quaintly resemble a bird's foot, even to the toes; whence the specific name of one species, Ornithopodioides; those of Hippocrepisremind one of a horseshoe; those of Trapa bicornis have an absurd resemblance to the skeleton of a bull's head. These likenesses appear to be accidental, but there are some which probably are of use to the plant. For instance, there are two species of Scorpiurus (Fig. 23), the pods of which lie on the ground, and so curiously resemble the one (S. subvillosa, Fig. 23, a) a centiped, the other (S. vermiculata, Fig. 23, b) a worm or caterpillar, that it is almost impossible not to suppose that the likeness must be of some use to the plant.
Fig. 23,— a, Pod of Scorpiurus subvillosa; b, Pod of Scorpiurus vermiculata.
The pod of Biserrula pelecinus (Fig. 24, a) also has a striking resemblance to a flattened centiped; while the seeds of Abrus precatorius, both in size and in their very striking color, mimic a small beetle, Artemis circumusta.
Mr. Moore has recently called attention to other cases of this kind. Thus the seed of Martynia diandra much resembles a beetle with long antenna: several species of Lupins have seeds much like spiders, and those of Dimorphochlamys, a gourd-like plant, mimic a piece of dry twig. In the common castor-oil plants (Fig. 24, b), though the resemblance is not so close, still at first glance the seeds might readily be taken for beetles or ticks. In many Euphorbiaceous plants, as, for instance, in Jatropha (Fig. 24, c), the resemblance is even more striking. The seeds have a central line resembling the space between the elytra, diverging and slightly diverging at the end, while between them the end of the abdomen seems to peep; at the anterior end the seeds possess a small lobe, or caruncle, which mimics the head or thorax of
|Fig. 24 a.—Pod of Biserrula.||Fig. 24 b.—Seed of Castor-Oil (Ricinus).||Fig. 24 c.—Seed of Jatropha.|
the insect, and which even seems specially arranged for this purpose; at least it would seem from experiments made at Kew that the carunculus exercises no appreciable effect during germination.
These resemblances might benefit the plant in one of two ways. If it be an advantage to the plant that the seeds should be swallowed by birds, their resemblance to insects might lead to this result. On the other hand, if it be desirable to escape from graminivorous birds, then the resemblance to insects would serve as a protection. We do not, however, yet know enough about the habits of these plants to solve this question.
Indeed, as we have gone on, many other questions will, I doubt not, have occurred to you, which we are not yet in a position to answer. Seeds, for instance, differ almost infinitely in the sculpturing of their surface. But I shall woefully have failed in my object to-night if you go away with the impression that we know all about seeds. On the contrary, there is not a fruit or a seed, even of one of our commonest plants, which would not amply justify and richly reward the most careful study.
In this, as in other branches of science, we have but made a beginning. We have learned just enough to perceive how little we know. Our great masters in natural history have immortalized themselves by their discoveries, but they have not exhausted the field; and, if seeds and fruits can not vie with flowers in the brilliance and color with which they decorate our gardens and our fields, still they surely rival—it would be impossible to excel them—in the almost infinite variety of the problems they present to us, the ingenuity, the interest, and the charm of the beautiful contrivances which they offer for our study and our admiration.—Forthnightly Review.
- Continued from page 171.