Chapter X: Dispersion of Fruits and Seeds

I purpose in this chapter to explain some of the many remarkable ways in which plants are enabled to scatter their fruits and seeds. The chief agencies which assist in carrying out this purpose are wind, animals, birds, running water, and moisture in the atmosphere. We shall find that many seeds are furnished with certain outgrowths and peculiarities which are specially adapted to the action of these agencies, with the result that such seeds are distributed far and wide. We will first examine some of those fruits which are scattered by animals; this end is generally attained by means of hooks and curved spines on the outside of the fruit.

Perhaps one of the most remarkable instances of this class is the seed-pod (or capsule) of the Martynias. During the visit of the Prince of Wales to India, a panther killed in one of the shooting excursions was found to have quantities of long-hooked seeds attached to his skin: these must have been brushed from a plant of _Martynia proboscidea_, which has sharp curved horns three or four inches long.

Another species called by the Italians _Testa di Quaglia_, or quail’s head, sows itself in a similar manner by clinging to moving objects.

Many common hedgerow plants have their fruits armed with quite formidable hooks, so that creeping or flying creatures may be made unwittingly the means of distributing the fruits. The burdock is a most persistent plant in this respect, each of its numerous fruits being covered with long hooks which successfully retain their hold of our clothing if we happen to brush past the plant when covered with its troublesome burrs. Other examples are the rough seeds of the forget-me-not, agrimony, enchanter’s nightshade—a great pest in gardens—and all the bedstraw tribe.

These plants, we may observe, are low-growing and herbaceous, quite distinct in the matter of position from the tall trees and shrubs which depend upon the wind to scatter their seeds.

We are all familiar with the winged fruits of the sycamore; they are to be seen in early autumn. The clusters are first of a pale green, and then the seeds[20] often attain a flush of pale crimson which adds much to the picturesque beauty of the tree. The equinoctial gales separate the seeds from their stalks, and away they go far and wide, borne up by the delicate membrane attached to the seed which catches the wind, and is carried by it to a great distance from the parent tree. In the same way the winged keys of the ash, being very light, are borne by the autumn gales to strange habitats, so that the tree may often be found growing on church towers, in ruins, and on crags inaccessible to man.

[20] In botany the fruit of the sycamore, maple, ash, &c., is called a _samara_, and is properly speaking a winged _achene_.

_Natural Size._ _Magnified._

BIRCH SEED.]

The pinus tribe of trees have seeds with wings slightly twisted so that, if we hold up a dry fir-cone, the seeds descend from it with a whirling motion like small shuttlecocks.

The winds which blow strongly in mountainous places carry these seeds before them, and are thus ever renewing the pine-forests by sowing the products of their cones on bare tracts of land. The lightest of all tree seeds is that of the birch; it is gifted with two wings or membranes, so that it floats in the air before the lightest breeze, and this may account for the wide distribution of the tree which has been found growing from Mount Etna to Iceland and Greenland. I may give an instance of a common which, twenty years ago, was covered only by furze, broom, and brake-fern; about fourteen years since, a shower of birch seed must have been strewn over the ground, and now it has become a wood, shutting out the distant views and quite altering the character of the landscape.

The wind again is the agency for the dispersion of the seeds of such plants as the common groundsel; here it may not be uninteresting to note the beautiful provision made in regard to the buoyancy of the seeds. These winged structures which the wind so lightly blows into the air must attain a certain altitude from which they can be successfully launched, and therefore we find that a large class of low-growing plants have their seeds furnished with accessories in the form of light silky down or hairs.

Most of the plants known as _compositæ_ have their seeds thus feathered, and amongst them are those plagues of the farmer, the thistle, dandelion, goat’s-beard and others. The dandelion may serve as our example, and I would advise my readers to watch carefully the variations of position in the flowering stems. Whilst the flower is still fully expanded the stalk remains in an upright position so that it is conspicuous and likely to attract the notice of insect visitors. After the florets are fertilised it gradually lowers itself until it lies on the ground under the leaves for a period of ten or twelve days. During this time the seed-vessel matures and ripens, then the stalk rises to the erect position once more, and the beautiful downy globe expands into a soft fluffy ball of seeds hanging so loosely that the first breeze carries them away, and their descent into the ground is curiously provided for. Persons have sometimes alighted on the earth from a balloon by means of a parachute, a machine which closely resembles an open umbrella with a car at the lower end. Now the dandelion seed descends in a similar manner, touching the ground first with its lower end, the weight of the seed enabling it to drop into some hole in the soil, and the spiny projections at the upper end preventing the feathery part of the seed from dragging it out again. The common goat’s-beard is perhaps the most beautiful English example of fruit with a downy pappus. A single flower will produce a sphere as large as a cricketball, and each seed is furnished with a starlike crown of branched feathers which the wind can bear away to a considerable distance.

The handsome willow-herb, which adds so much colour and beauty to our river banks, bears its seed in long, narrow pods, and these, when ripe, split up into five segments which, curling back as they open, leave the downy seeds free to be carried off by the passing breeze.

Bird agency in seed dispersion is a most interesting subject, and one can but admire the wonderful way in which the services of winged creatures are made available.

Succulent berries and sticky fruits are highly attractive to many kinds of birds, and whilst they revel upon the sweet, soft flesh of the berry, the seeds which they swallow with it are enabled to resist the action of digestion by a hard covering which protects the kernel until the shell shall decay and allow the seed to germinate. In this way I find my garden in early spring quite thickly strewn with the seeds of the Irish ivy, always a favourite food of the common wood-pigeon which is so frequently to be heard cooing in my woods.

The seeds of aquatic plants often cling to the feathers of birds that visit pieces of inland water, and are widely distributed by them in their flight from one lake to another.

Darwin has shown by careful experiment that the mud clinging to the feet of various birds almost always contains seeds. A wounded partridge had a ball of earth weighing six and a half ounces adhering to its legs. From this earth Darwin reared no less than eighty-two separate plants of five distinct species. Seas and rivers also do their part in dispersing seeds. The huge nuts of the _Cocos-de-mer_ palm, which grows only upon the Seychelles Islands, are often thrown upon very distant shores. This nut is said to take ten years to come to perfection; it is exceedingly hard, and sometimes weighs as much as forty pounds. The common cocoa-nut is often found growing on the shores of coral and other islands in the Pacific Ocean, and owes its position there to the buoyant nature of the nut, which floats uninjured in the sea until it finds a resting-place and a home on some atoll or island. In this way the cocoa-palm has spread to such an extent that it is now perhaps the only palm common to the western and eastern hemispheres. West Indian seeds and fruits have even been thrown upon the Norwegian coasts, and, but for the unsuitability of the climate, there is little doubt that tropical trees and plants might sometimes be found growing even so far north. It is obvious that the seeds of all vegetation on the banks of rivers, small running streams and lakes, must be liable to very wide distribution. Darwin made many interesting experiments as to the length of time seeds could retain their vitality when floating in fresh or salt water. Ripe hazel nuts germinated after being ninety days in water. An asparagus plant with mature berries, when dried, floated for eighty-five days, and the seeds afterwards grew vigorously. Out of ninety-four plants experimented upon, eighteen floated for more than a month and some for three months, their germinating power not being destroyed. In quite a large number of species the plants themselves possess the means necessary to distribute the seed. It is true the distance traversed by each seed may not be great, but it is sufficient to give the seed a new field of growth. This power varies in different species. It is perhaps best defined as elastic force, and in the majority of cases the seed is actually thrown away from the parent plant by the expenditure of this force. The seed-pod is generally in a state of tension, due to the gradual drying up of the tissues. Then a puff of wind, a slight blow, or even a change in the atmospheric condition of the air, gives the final impetus, causing the pod to burst with such force that seeds are thrown out in all directions. The fibro-vascular cords are often found crossing the pod in an oblique direction, or even in a spiral manner, so that finally, as they shorten through dryness, they act upon the walls of the legume and we see the result in such dried pods as those of the sweet pea, broom, and laburnum.

The pansy has a three-valved seed-pod, and as it dries the edges of the valves press upon the polished, hard-shelled seeds and they are squirted out with a jerk to a distance of several feet. I was once greatly puzzled by a strange, crackling sound in my room, and after a few minutes’ search I discovered it was caused by a fusillade of pansy seeds striking against the sides of a small box in which I had placed the capsules to ripen. It is worthy of notice that the capsule hangs down to protect the seed-valves from rain; but when the seeds are matured the capsule rises to an upright position so that they may be projected far and wide. A conspicuous example of the elastic force of which I have spoken is seen in the British balsam, _Impatiens Noli-me-tangere_ (touch-me-not). When its seeds are mature, the valves of the capsule curl up in a spiral form with such force as to project both themselves and the seeds through the air many feet from the plant dropping the seeds by the way. On a hot summer’s day one may hear the dispersion of seeds! The furze and broom pods, the sweet peas, and especially fir tree cones, make quite a loud report as they split and scatter their contents. The tension causing these explosions is in some cases brought about by the fluids inside the fruit. This is the case with the squirting cucumber, which, when fully ripe, is so distended with fluids that the slightest touch or movement is sufficient to cause it to break away from its stalk, and then the whole contents are ejected with great force, so that the seed is thrown some distance. The extent of dispersion is very limited in those plants that are dependent upon the varying moisture of the air. Such plants are usually furnished with special awn-like[21] appendages; these are hygroscopic[22] in their nature, and the difference in the amount of moisture in the air lengthens and contracts these apparently moving organs. When the seeds fall from an ear of barley they lie thickly strewn around the bottom of the stem, and, were they to take root there, they must inevitably choke each other; but each awn is thickly set with bristles, and as the morning sun shortens and the evening dew lengthens the hair-like awn, the prickles only allow the awn to move in one direction, and the seed which is attached to it is slowly but surely drawn many inches away. What is popularly called the dancing oat is another curious example of this hygrometric property. If a dry seed (or oat) is placed for a moment in water, and then laid on a smooth table, it will be seen to wave its long horns as if they were the antennæ of an insect, and to turn over and over until it has progressed some inches from the point where it was first placed. In _Avena elator_ (the tall oat grass) and _Stipa pinnata_ the awns are bent sharply just as they emerge from the flowers, the part below the bend being like a corkscrew and highly sensitive to moisture, relaxing and contracting according to the amount of moisture in the air, with the result that the seed travels along the ground. By the help of the long awn it can pass over small obstacles, such as stones or clods, the movement resembling that of a lever.

[21] The beard of corn.

[22] Sensitive to moisture.

I must here guard my readers against those movements that are caused by some insect larva. The so-called jumping bean imported from Mexico is now so well known that it may be taken as a type of these curious movements due, not to the seed itself, but to the efforts of an imprisoned insect, the grub of a small moth which passes its larval stage inside the hard-shelled seed of a kind of euphorbia.

In conclusion, we may glance at a small group of plants that develop sticky glands for the purposes of dispersion.

That charming Alpine plant _Linnæa borealis_ has a pair of bracts closely adherent to the fruit and these bracts are covered with stalked glands of a sticky nature, so that when an animal, bird, or even a passing moth brushes against the little fruits they stick to the intruder and are thus borne away. Now it may perhaps occur to the thoughtful reader that the Linnæa seed-vessel, being part of a growing plant, would not readily break off with a slight touch, but it is another instance of that consummate skill and arrangement that is so apparent to the close observer. In the stalk of the little fruit there is a special separating layer[23] (analogous to that of the falling leaf which we noted in a previous chapter), and at this point the fruit readily separates if the slightest pressure is brought to bear upon it. This example is typical of what takes place in such plants as _Salvia glutinosa_ and _Plumbago capensis_ and _Rosea_. As a contrast to these various modes of dispersion I may mention those seed-vessels which are actually buried by the plants themselves, such as the ground-nut, ivy-leaved toad-flax, and others. We must bear in mind that these plants usually have aerial flowers in addition to those matured underground, and that these aerial flowers produce fruits which are subject to dispersion. We may therefore conclude that the underground seeds are to ensure the continuance of the plant when the ordinary methods have perhaps partially failed. My readers may each autumn find an endless source of wonder and interest in the thousands of differing fruits and seed-vessels which may be obtained in any hedgerow and field; and by careful observation they may yet learn many new facts and be ever adding to their store of knowledge by gathering and comparing the fruits and their dispersion, as shown in the types sketched in this chapter.

[23] Called botanically an “absciss layer.”

Objects to collect and examine:—Fruits and seed-vessels of martynia, burdock, forget-me-not, agrimony, enchanter’s nightshade, bedstraw, samara of sycamore, ash-keys, pinus seeds, birch seeds, dandelion and goat’s-beard seeds, pods of sweet-pea, broom, laburnum, and pansy. Seed-vessels of balsam, wild oat, feather-grass, _Linnæa borealis_, salvia, plumbago, ground-nut, and ivy-leaved toad-flax.

CHAPTER XI

_GERMINATION_

“O Source unseen of life and light,
Thy secrecy of silent might
If we in bondage know,
Our hearts, like seeds beneath the ground,
By silent force of life unbound,
Move upward from below.”

+T. T. Lynch+