FRUITS for protection and dispersal

The ovary undergoes special changes as as a result of fertilization, gradually forming that familiar part of the flowering plant—the fruit. Essentially the fruit is the container for the seeds, and it develops side-by-side with them, affording protection from both mechanical damage and the effects of the elements. By the time the seeds are mature and ready to be dispersed, the fruit is ready to play its part too: the ripe fruit is specially adapted to secure dispersal by one method or another so that its seeds may be spread over as wide an area as possible. Sometimes, the piilp of the fruit contains chemical substances which control the dormancy of the seeds. This occurs in the tomato, pear, apple and lemon, and the seed, to escape its dormancy, must be physically separated from the fruit. The fruit is thus a very important structure, and the success of the flowering plants, so obvious today, owes much to its existence. ‘True’ fruits and ‘false’ fruits

The word ‘fruit’ really only applies, in the strict botanical sense, to the structure resulting from the female part of the flower, the gynoc-cium, alone. A true fruit would thus consist of a muchenlarged ovary with perhaps the remnants of the style and stigma, shrivelled but still attached. However often other floral parts, besides the gynoecium, have gone into the make-up of a fruit. For example, in the apple the fleshy edible part is in fact developed from the receptacle of the flower, and the true fruit lies further inside, forming the core with its pips being the seeds. This fruit is called a ‘pome’ and is typical also of the hawthorn, quince and pear. A much-swollen receptacle is also responsible for the fleshy part of the strawberry, in this case the pips being the true fruit. The fig is a strange and complicated structure. A single fruit is derived from a mass of little flowers which become covered by a succulent container formed from the inflorescence stalk. The pineapple again includes a whole inflorescence on a fleshy stalk in its massed and succulent fruit (all the floral parts being included). Such fruits are called false fruits, or pseudocarps.

An interesting point concerning the greengrocer’s distinction between fruit and vegetables is that many of his so-called ‘vegetables’ are in fact true fruits, such as the tomato, the marrow, French and runner beans, and the cucumber.

There are two principal lines along which a fruit may develop. One leads to the formation of a hard dry tough exterior, the other to a soft fleshy succulent form. The exact form depends largely on the original structure of the ovary, which can vary considerably.

Berries and drupes

Fleshy fruits are of two major sorts: berries and drupes. The ovary wall or pericarp of a berry is fleshy all through, as in the tomato. It has a firm skin called the epicarp surrounding the central succulent mass, which is the mesocarp. The seeds lie in the centre of the berry, being the ‘pips’ in the tomato. The banana, too, is a berry. It usually will be found to have no seeds because it is commercially developed that way, but in the centre of the fruit are some small brownish objects —the remains of the ovules. Citrus fruits, such as oranges, lemons and grapefruit, are berries in which the mesocarp as well as the epicarp has become firm and leathery, and the carpels themselves, fused in a compound ovary, form the fleshy edible part. If the skin (epicarp) and the bitter white part under the skin (the mesocarp) are peeled away the juice- packed carpels can be separated out with a knife.

A drupe, on the other hand, has its pericarp in three layers, not two. The epicarp is firm, the mesocarp fleshy, as in the berry, but the third layer, the endocarp, is very hard and stony. This endocarp surrounds the typically single seed, as in the plum, a typical drupe. The plum stone is not the seed: it is the stony endocarp, and when it is broken open, the real seed is inside. The walnut (Juglans regia) and the coconut (Cocos nucifera) are both drupes, but they are sold in shops with the cpicarp and mesocarp stripped off, leaving only the hard endocarp. The nut kernel is the seed. Quite often a lot of little drupes are gathered together on a receptacle to form a fruit such as the raspberry or the blackberry.

Berries and drupes constitute an important part of commercial fruit production.

Dry fruits

In many fruits the pericarp does not become fleshy; instead it becomes toughened and dry, and sometimes woody as well. When they are mature these dry fruits may split open to disperse their seeds (dehiscent) or they may not (indehiscent). When a single carpel enclosing a single seed becomes an indehiscent fruit, it is termed an achene. Examples of achenes are found amongst the buttercups, the wood avens (Getim urhanum) and clematis.

A nut is usually an achene too. A hazelnut, for example, has a hard woody shell, the pericarp of the fruit. Now, if the nut is cracked open, out falls the single seed, the part which is eaten. Many other nuts are achenes too, including the fruits of the sweet chestnut (Castanea), beech (Fagus), and oak (Quercus). Exceptions are the walnut and coconut which are technically drupes.

Most achenes have a separate seedcoat and fruit wall, but sometimes, as in the grains of cereals and other grasses, the two become fused together, forming what is known as a caryopsis. Other achenes have their pericarps produced into a wing as in ash and elm, whilst the sun- flower, being typical of its family, the Com-positae, has a sort of false-fruit-achene as its pericarp is partially derived from the receptacle. A fruit formed from collections of little achenes is quite a common type; it can be found in the buttercups and also in the strawberry where the achenes develop as the little ‘pips’ on the expanded receptacle.

The forms of dry dehiscent fruits are varied. The capsule, a common type, develops from carpels which are fused together (a syncarpous gynoecium) and opens by splits, pores, or teeth, as in many of the wallflower family, the Cruciferae.

Distinctions between a fruit and a seed

Isolated from the plant, a fruit is sometimes difficult to distinguish from a seed. Remember that a fruit is a fully-ripened gynoecium, which should therefore hold seeds inside it, and look for these. On the outside, a fruit will usually show the flower stalk by which it was attached to the plant, whereas the seed will show a scar called the hilum, the position of its detachment from its ovary stalk. The withered remains of one or more styles are visible on the fruit, but these are never to be found on a seed. Sometimes a seed shows a little hole or pore, called the micropyle, but no such hole is borne by a fruit. Upon opening, the differences revealed are more striking. A many-seeded fruit has its pericarp enclosing the seeds, which are separate from each other and the pericarp. Difficulties may arise where the fruit is one-seeded, as in an achene, particularly if it is also very small. The outer covcring(s) has to be peeled off to display the embryo. One layer usually indicates a seed, though the caryopsis, with its fused pericarp and seedcoat, must first be ruled out. If two layers have to be removed, it is safe to assume that it is a fruit, the two layers being the fruit pericarp and the seed testa. Sometimes a third layer is present. This is probably due to a cupulc which surrounds the fruit until it is mature, and in such a case would be responsible for the outermost layer.

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