The fruit as an agent of seed dispersal

Although the fruit serves first and foremost to house and protect the developing seeds, it also plays a secondary, but also very important, role of securing the dispersal of the seeds. In most plants it is the fruit which is the main agent of dispersal. Wind dispersal

Wind dispersal is very common, both among fruits and seeds, and there are numerous modifications to be found associated with this method. Both fruits and seeds can develop a very similar mechanism for aiding wind dispersal. Wide dispersal can be achieved, for instance, by having very lightweight fruit, like seeds, which can take to the air with great case. Such fruits are often called ‘dust fruits’, and a good example of this is the exotic wormwood, Balanophora, Other fruits become winged, plumed or woolly, again like seeds. The fruit of the elm (Ulmus) is an achene, and its pericarp grows out to form a membranous wing. This can act as a sail or propeller to catch the wind. In the ash (Fraxinus excelsior) the fruit is a pair of achenes, each of which has a long twisted wing which sends the fruit spinning on its way. The sycamore and the maple are also similarly dispersed. Some fruits have wing structures which develop from parts other than the carpel wall; in the hornbeam and hop fruits, wings develop from bracts, while in species of Salvia the calyx may form a sail. The fruit of the dock is covered by the calyx which develops three wings.

Usually the stigma and style of a flower wither away soon after fertilization but sometimes they can persist and come to aid in dispersal. In traveller’s joy (Clematis vitalba) the style remains and becomes feathery to catch in passing breezes. This plant is called ‘old man’s beard’ by some because of the grey fluffy appearance of its fruits. Many members of the Compositae, the daisy family, develop a ring of hairs, called the pappus, in place of the calyx in each fertilized floret. The dandelion develops a long stalk on top of the ovary to carry the hairy pappus at its end, whilst the remainder of the fruit develops into an achene. A ripe dandelion ‘clock’ thus consists of a collection of little achenes, each with their pappus and stalk, positioned on the flower base, a structure which starts out as being flat, but curves in drying so that the fruits assume a spherical arrangement. This positioning is ideal for dispersal by wind, the slightest puff of which lifts and carries the fruits away like tiny parachutes, often to great distances.

Woolly fruit, for example the fruit of the pasquc flower, function in a similar wind-catching manner to those with wings and plumes.

In steppe and other areas which tend towards desert conditions, a type of wind-dispersed plant known as a tumble-weed is common. Here, where vegetation tends to be low-growing and wind velocities high, the whole plant or a large chunk of it can be blown along by the wind as a regular method of dispersal. Elsewhere, some kinds of grasses and seaside plants such as the sea bladder-campion (Silene maritima), constantly subjected to strong winds in an open habitat, are dispersed by a slightly less extreme version of this tumble-weed method in which the seed-bearing flower head in its entirety is blown off by the wind. Animal dispersal

Dispersal of fruits by animals is very common and occurs either by the animal eating the fruit and later voiding the seeds, or by the fruit being carried off on its feet or hide. In fleshy fruits, the succulent portion is usually specially modified for catching the animal’s eye. Until ripe, the fruit stays green, merging inconspicuously with the foliage. As it ripens, the fruit develops a bright conspicuous colouration, frequently in shades of red to contrast with the green leaves. At the same time the fruit develops its characteristic appealing odour and attractive texture. These fruits attract a variety of animals. In temperate regions, birds are the chief consumers, but in the tropics, monkeys, wild pigs and fruit bats are amongst the assortment of creatures which visit plants for their fruits. In feeding, the animals either discard the seeds, well-protected in their tough seedcoats as in berries and achenes or inside stony endocarps of drupes, or swallow them. In the latter case the seeds pass through the gut system and out with the excreta, generally unharmed and sometimes in a better condition for germination. Quite often a bird will gorge itself on its favourite fruit, fly away and then eject the pulped mass, apparently suffering from the effects of over-eating!

Fruits can be carried externally on an animal’s body, and common adaptations to this mode of dispersal are hooks and spines, bristles and mucilage, any of which may cause the fruit to stick to fur or feathers of a passing animal. Almost any part of the fruit wall can become modified in this way; in the wood avens and the buttercups, it is the style which becomes hooked, in the carrot, woodruff’ and goose-grass, it is the ovary wall, whilst in the forget-me-nots (Myosotis species) and bur-marigolds {Bidens species) the fruiting calyx develops hooks.

In plants which rely on animal dispersal, we find the dispersal structures developed are generally fewer and less complicated than those developed as adaptations to that other major dispersal agent—the wind. This is because animals tend to frequent areas which are fertile, so that fewer seeds end up on barren ground. The plant can thus ‘afford’ to minimize its expenditure on reproductive devices. According to the type of animal involved, dispersal of fruit by animal means can be quite far-ranging. This is particularly true with birds, and also with the fruit bats which have been known to travel 48km (30 miles) on nocturnal fruit-foraging expeditions for mangoes, guavas and bananas, returning to their roosts the same night.

Of course, animals can also disperse fruit in other rather less-conventional means. Hedgehogs can accidentally spear fleshy fruits onto their spines and carry them considerable distances before dense undergrowth brushes them off again. Nut-eating animals like squirrels can make a cache and never find it again. Birds and other animals can carry fruits stuck to their feet. The Malayan plant Rajfesia arnoldi, a parasite on the roots of vines, has its fruits dispersed by elephants; the fruits are large and squashy so that they stick to the elephants’ feet and the seeds are carried and pressed down against the roots of the host plants. It seems that only an elephant has a tread which is heavy enough to press the seeds into the soft jungle soil where they can germinate up against the vine’s roots. Other methods of dispersal Many plants have self-dispersal mechanisms, but the fruit itself does not become dispersed with the seeds.

Dispersal of fruits by water is rather uncommon. Many fruits, as with seeds, will float but they do not, remain viable for long under such conditions. Adaptations for water transport in the fruits of those plants which do rely on this method take the form of loose or spongy coverings which can retain pockets of air to give them buoyancy. The yellow water lily (Nuphar lutea) has a large berry fruit which, when ripe, splits into its component carpels, each of which is lined with mucilage containing air bubbles so that they can float away. Later the carpels decay, releasing the seeds which then sink to the bottom of the water where they germinate.

Perhaps the best example of a water-dispersed fruit, however, is the coco de mer or double coconut of the Seychelles (Lodoicea maldivica), which is one of the largest fruits known. This fruit, along with that of the coconut (Cocos nucifera), has a tough woody covering to protect the seed and a lot of air-trapping fibrous matting to give the fruit buoyancy during its passage in the ocean currents. These plants are often amongst the first to colonize oceanic islands or coral reefs because of their readily water-borne fruits.

Those plants which grow totally immersed in water rarely produce buoyant parts; many produce achcnes which sink quickly on being shed.

Mutiny on the Bounty

In 1787 the 215-ton vessel Bounty, commanded by William Bligh, left England for the specific purpose of transferring live breadfruit trees from their native Tahiti to the West Indies. A gardener from Kew, David Nelson, was responsible for collecting and growing the plants in special quarters. However, after eventually leaving Tahiti, the ship’s crew mutinied: Bligh, Nelson and 17 others were set adrift in a small boat, and the 1,000 breadfruit plants were thrown overboard. Though Bligh has been portrayed as a hard captain, the crew most probably rebelled against the occupation of limited cabin room by the outlandish plants and the unnatural work of tending them, including the use of precious drinking water. After a journey of 3,518 miles, Bligh brought the castaways to Timor, where unfortunately Nelson died of a fever. In 1791-3 the breadfruit expedition was repeated, again under Captain Bligh with two Kew gardeners, and a shipload of breadfruit and other useful Tahitian trees was safely brought to famaica.

Fruits and man

Of course, fruits can be dispersed by chance, by-passing the ‘set’ methods available by animals, wind or water. Not the least of these chance agencies is man, for whom fruit has held a strong attraction the world over since Adam and Eve. Too strong an attraction for some, perhaps, since the deaths of Pope Paul II in 1417, the German King Frederick the Peaceful in 1493 and his son Maximilian in 1519, were all supposedly caused by a surfeit of melons. In AD 200, the Gracco-Roman ‘prince of physicians’, Galen, expressed the opinion that his father had lived to reach the age of 100 because he never ate fruit. Deprived years, wc might well consider today, when fruit plays such an important part in our diet, both from gastronomic and nutritional points of view.

All over the world, fruits of many kinds are being cultivated on a large scale. Many of the wild ancestors of our cultivated fruits bear fruits which, by today’s standards, seem small and inconspicuous in comparison. The wild banana (Musa rosacea), for instance, bears banana fruits which have little pulp and are barely edible. Over the years careful selective breeding has produced plants bearing bigger and better crops than ever before, and varieties to suit almost every kind of demand, which are often seedless into the bargain. This is particularly true of that most beloved of all fruits— the apple. According to Genesis, the apple (Mains pumila) was created three days before man, but, unlike man, it has never fallen from grace; today it is the world’s most cultivated tree fruit. Varieties of apple abound; some of them have become deservedly famous, such as the Cox’s Orange Pippin and the Golden Delicious.

Besides being a pleasure to cat, fruit also provides an important source of vitamins and minerals in the human diet. British sailors on long voyages used to suffer from scurvy, a severe skin disease caused by a vitamin de-deficiency until it was discovered that upon supplementing their restricted diets with fresh citrus fruits, the symptoms disappeared. Hence the British sailors came to be known by the Americans as ‘limeys’ on account of their habit of hoarding fresh limes on board during long voyages. Large amounts of vitamin C provided by citrus fruits are thought today to give resistance to the common cold.

Vegetative propagation of plants

In some plants, reproduction by means of fruits and seeds produced by the sexual process does not occur, and the only way in which they can manage to reproduce themselves is by the so-called vegetative method. Whole new plants grow from plantlcts or bits and pieces derived from an already established plant. Since no genetic variability is involved, these new plants are just like the parent plant in all their characteristics. Sometimes plants are propagated vegctatively on a commercial basis because of this feature, as it enables a desirable character to be preserved whereas with seed planting, it could be lost in genetic juggling. In nature, this vegetative propagation results from several methods. In the grasses and cereals it results from the rooting of stems from tillers whilst in the strawberry, runners are produced. Other plants use all sorts of bits and pieces. The Canadian waterweed can grow from any part severed from the parent plant and consequently can rapidly establish itself over large areas of water. Irish potatoes are grown from pieces with eyes, which are tiny dormant plant shoots, whilst in horse-radish buds develop from pieces of root. Buds also develop on leaves in some plants; a whole African violet plant can be grown from just a leaf with its base or stalk placed in water or earth.

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