Fruit

Formation of fruit : Fruit is defined as fertilized ovary. The ovary develops into fruit. The ovary wall at maturity forms the wall of the fruit, which is known as pericarp. Sometimes, other parts of flower such as tepals, (e.g., Morus), bracts (e.g., Ananas) or thalamus (e.g., Pyrus) are also involved in the formation of fruit and such fruits are called false fruits or pseudocarps.

The fate of various parts of the ovary during the formation of fruits is summarized below :

Types of fruits

They are classified into three groups : Simple, aggregate and multiple or compound fruit.

Simple fruits : They are formed from mono-or polycarpellary but syncarpous ovary. They may be dry or fleshy.

(1) Simple dry fruits have thin, hard and dry, pericarp. They are of three kinds :

(i) Dehiscent or Capsular (ii) Achenial or Indehiscent (iii) Schizocarpic

(i) Dehiscent fruit : These fruits are dry, many seeded and split open at maturity. They are of following types :

Legume or Pod : It is characteristic of the family leguminosae developed from monocarpellary unilocular superior ovary with marginal placentation. It can open or dehisces by both ventral and dorsal sutures. e.g., in Cicer arietinum (Gram) Pisum sativum (Pea) and Phaseolus mungo (Black gram).

Follicle : It is very much resembles the legume but on ripening it opens generally along the ventral suture. e.g., Calotropis, Larkspur, etc.

Siliqua : The fruit is developed from bicarpellary, syncarpous and superior ovary which bears ovules on two parietal placenta. The ovary is unilocular but later becomes bilocular due to the development of a false partition wall called replum. It dehisces from the base towards the apex by both the sutures e.g., In Brassica (Mustard) and is characteristic of the family Cruciferae.

Silicula : It is flattened and short in length from siliqua type, found in Iberis (Candytuft) and Capsella bursa (Shepherd’s purse).

Capsule : It is mono or polycarpellary, dry dehiscent, many seeded fruit which developes from a superior or inferior ovary. It dehisces in almost all the ways i.e., longitudinal and transverse, along both the sutures. Majority of capsules show longitudinal-dehiscence which again are of different types :

Loculicidal : Lines of dehiscence appear along the dorsal sutures, e.g., Gossypium herbacium (Cotton) and Abelmoschus esculentus (Lady’s finger).

Septicidal : Lines of dehiscence appear along the ventral sutures or septations of the ovaries e.g., in Viola (Pansy), Linseed (Linum).

Septifragel : Lines of dehiscence along irregular lines, but the seeds remain attached to the placenta, as in Datura stramonium (Thorn apple).

(ii) Achenial or Indehiscent fruits : These fruits do not burst at maturity but the seeds are liberated only by the decaying of the pericarp. These are of following types :

Achene : It is small, dry one seeded fruit which develops from a superior or inferior monocarpellary ovary. In this type, the pericarp is tough but thin and free from the seed coat, e.g., in Mirabilis (four o𠆜lock plant) and Clematis. Some times achenes occur in a group from apocarpus ovary where carpels are many e.g., Nelumbium (Lotus).

Caryopsis : It is very small, dry and one seeded fruit which develops from a superior monocarpellary ovary. Here the pericarp is closely fused with seed coat. It is the characteristic of family graminae, e.g., Oryza sativa (Paddy), Triticum aestivum (Wheat) and Zea mays (Maize).

Cypsela : It is dry, one seeded fruit which develops from an inferior , bicarpellary ovary. Here the pericarp is free from seed coat but the thalamus is fused with pericarp. The fruit is provided with a crown of hairs at the top called pappus e.g., in Helianthus annuus (Sun flower), Tridax, Cosmos, Sonchus, etc.

Nut or Glans : It is dry, one seeded fruit which develops from a superior, bi or polycarpellary ovary having a hard pericarp, free from seed coat e.g., in Areca catechu (Betalnut), Anacardium occidentale (Cashewnut) and Trapa natans (Water chestnut). Here the thalamus and sometimes the cotyledons of true fruit are also edible.

Samara : It is dry, one or two seeded fruit, develops from a single mono-or bicarpellary ovary. The pericarp is free from testa and produces a wing like outgrowth which helps in the dispersal of seeds e.g., in Hiptage and Elm.

(iii) Schizocarpic or Splitting fruits : These resemble both (achenial) indehiscent fruits as well as capsular fruits having many seeds. However, they break into one seeded segments known as mericarps. By splitting usually the mericarps are indehiscent but in Ricinus (Castor) they are dehiscent. The important schizocarpic fruits are :

Lomentum : It is a dry, many seeded fruits which develops from a monocarpellary, superior, unilocular ovary with marginal placentation.

The fruit arises just like a legume but when ripened it becomes partitioned between seeds into single seeded mericarps e.g., Acacia arabica (gum tree), Mimosa (touch me not) and Dalbergia sisoo (India red wood tree).

Cremocarp : It is a dry fruit, develops from bicarpellary, syncarpous, bilocular ovary. The fruit when mature breaks into single seeded mericarps which remain attached to the top of the central axis called carpophore, e.g., Daucus carota (Carrot) Foeniculum vulgare (fennel).

Regma : It develops from tri-or penta-carpellary superior syncarpous ovary. The locules are many as the carpels known as Cocci (sing. Coccus), attached to carpophore and separate by splitting e.g., Euphorbia, Geranium and Ricinus.

Carcerulus : It is a dry fruit, develops from bi or polycarpellary syncarpous, multilocular superior ovary with axile placentation. Many single seeded, mericarps are formed by splitting and formation of false septa. e.g., Ocimum sanctum (Sacred basil), Althaea rosea.

(2) Simple fleshy fruit :  The fruits are simple, but the pericarp is fleshy and edible. It is differentited into three layers epicarp, mesocarp and endocarp. Fleshy fruits are of following types:

Drupe : It is a fleshy fruit formed from mono-or poly carpellary superior ovary, where one or more ovules may developes into seeds. Here the epicarp is thin and leathery. The mesocarp is thick, fleshy, juicy and edible in Mangifera (Mango) and fibrous in Cocos (Coconut).

The endocarp is hard and stony in both the cases. In Cocos, pericarp is not edible. The portion inner to endocarp is the liquid endosperm which is edible.

Berry : It is usually many seeded fleshy fruit develops from polycarpellary, syncarpous, superior ovary. Rarely it is single seeded as in Borassus (Palm).

Here the epicarp remains as the skin of the fruit. The mesocarp and endocarp are fused together to form the pulp of the fruit. e.g., Brinjal, Tomato, Banana, etc.

Pepo : It is a special type of berry. Here the epicarp and thalamus form the outer ring of the fruit. The mesocarp, endocarp and placentae are fused to form pulp which is edible seeds are many. Common examples are Cucurbita maxima (Sweet gourd) Cucumis sativa (Cucurbit).

Pome : The fruit develops from inferior, pentacarpel ovary. The fruit is covered by the fleshy thalamus, which is fused with the pericarp and edible. The outer part again encloses the inner stiff and membranous portion enclosing the seeds common example is Pyrus indica (Apple).

Hesperidium : It is another type of berry it develops from a polycarpellary, syncarpous, superior ovary with many seeds. Here the outer skin is thick and leathery that represents the epicarp, which contains oil glands. The fibrous portion fused with epicarp is the mesocarp. The endocarp consists of many chambers with juicy glands. Common examples are Citrus medica (Lemon) and Citrus sinensis (Sweet orange).

Balausta : This is many chambered, many seeded fruit developing from a multicarpellary, syncarpous but inferior ovary. The pericarp of balausta is leathery or tough. The carpels are arranged in two rows. Carlyx is persistent. The seeds have succulent seed coat (testa) which form the edible part e.g., Punica granatum (Pomegranate).

Aggregate fruits : The aggregate fruits are formed from polycarpellary, apocarpous ovary. Each ripened is called fruitlet or etaerio e.g., the lotus, rose fruit and strawberry are a collection of achenes raspberry, a collection of drupes and custard apple is a collection of berries.

Composite or compound fruits : Multiple fruit develops from entire inflorescence called sorosis or syconus.

Sorosis : Develops from spike or spadix inflorescence e.g., Pineapple, Jackfruit, Mulberry, etc.

Syconus : Develops from hypanthodium inflorescence e.g., Ficus carica. (banyan).

Seed

Development of seed : The fertilized ovule forms seed. The ovule increases greatly in size. The integuments dry up. The outer one becomes hard or leathery and forms the outer seed coat or testa while the inner one, if persist, forms the tegmen.

The nucellus is generally used up during the development of embryo but in some cases it remains outside the endosperm in the form of a thin layer, called perisperm. The endosperm may persist or completely digested during embryogenesis.

A scar is usually visible on one side of the outer seed coat. It is known as hilum and marks the point of attachment to the stalk. With these changes, the ovule changes into seed and enters a period of dormancy while the ovary ripens into a fruit.

Dicotyledonous seeds

Exalbuminous : Gram, Pea, Bean, Mustard, Mango, Groundnut, etc.

Albuminous : Castor, Poppy, Artabotrys, Custard apple (Ananas) etc.

Monocotyledonous seeds

Exalbuminous : Orchids, Alisma, Najas, Pothos, Amorphophallus, Vallisneria, etc.

Albuminous : Cereals, Millets, Palms, Lilies, etc.

Non-endospermic or Exalbuminous seeds : In exalbuminous seeds endosperm is completely consumed by the developing embryo, and the mature seeds are without endosperm. The food is stored in cotyledons.

Endospermic or Albuminous seed : In albuminous seeds, embryo not consumed all endosperm. So it persists in the mature seed. In these seeds food stored in endosperm. In monocot seed the membranous covering of :

$\bullet$ Radicle is called coleorrhiza.

$\bullet$ Plumule is called coleoptile.

Germination of seeds : The process by which the dormant embryo of the seed resumes active growth and grows into a new plant is known as germination.

Types of seed germination

Epigeal germination : In this type of germination, the cotyledons come above the surface of the soil into the air and light due to the rapid growth and elongation of the hypocotyl. The cotyledons turn green and finally dry up and fall off and seedling becomes an independent plant. Germination of seeds of Bean, Gourd, Castor, Cotton, etc. is of epigeal nature.

Hypogeal germination : In this type of germination, the cotyledons remain in the soil or just above the surface. In this case epicotyl elongates pushing the plumule upwards. The cotyledons do not turn green and gradually dry up and fall off. Common examples of hypogeal germination are the seeds of Pea, Mango, Groundnut, etc.

Viviparous germination : This is a special type of germination found in mangrove plants. The embryo grows not only out of the seed but also out of the fruit and projects from it in the form of a green seedling displaying root and hypocotyl. Due to its increasing weight the seedlings separate from the parent tree and falls into the mud or water and soon develops lateral roots. Vivipary is seen in Rhizophora and Sonneratia.

Factors for seed germination

External factors : Water, oxygen, suitable temperature.

Internal factors : Foods and growth regulators, completion of rest period, viability.

Seed dormancy : In several plants seeds germinate as soon as they have undergone maturation and provided proper conditions for germination. e.g., seeds of Bean, Pea, Maize etc. In some plants seeds are incapable of germination because of some inhibitory factors. Such seeds are unable to germinate even under suitable conditions. This is called seed dormancy.

Causes of seed dormancy

The seed dormancy may be due to many causes some of which are as follows :

(1) Impermeability of seed coats to oxygen. (e.g., Xanthium) and water. (e.g., Chenopodium and many leguminous seeds).

(2) Seed coat is mechanically hard, thus resisting the growth of embryo. e.g., Mustard, Capsella, Amaranthus.

(3) Presence of rudimantary or immature embryo. e.g., Ginkgo biloba (a gymnosperm).

(4) Some plants produce such chemical compounds that inhibit the germination of their own seeds. e.g., Tomato, (possesses inhibitor ferulic acid).

Dispersal of fruits and seeds

Dispersal by wind (Anemochory)

The wind is probably the most important agency of seed dispersal in nature. The fruits and seeds show following devices which help in dispersal by wind.

Light weight and minute seeds : Seeds of some plants (e.g., Orchids) are sufficiently light and minute in size to be easily carried away to great distances by air currents.

Winged seeds and fruits : Some seeds (e.g., Oroxylon, Cinchona, Moringa) or fruits (Acer, Hiptage, Terminalia, Dipterocarpus) develop one or more thin membranous wings to ensure their dispersal by wind.

Parachute mechanism : In members of the family Asteraceae (Compositae) e.g., Taraxacum, Sonchus, sepals are modified into tufts of hairs called pappus. The pappus is persistent and hence found attached to even small, single seeded fruits. It acts like a parachute that allows the wind to carry them to great distances. Seeds of many nasty weeds are also dispersed by this method.

Censer mechanism : In Antirrhinum (dog flower), Aristolochia, Papaver (poppy), Argemone mexicana (Prickly poppy), Nigella (love-in-a-mist), etc. the fruit is a capsule. At maturity it ruptures but the seeds do not come out. However, when the capsule is shaken violently by the wind, the seeds are scattered in all directions. In this process all the seeds do not escape together.

Rolling mechanism : In some species, like Amaranthus albus, Chenopodium album, etc., plants dry out after bearing fruits and seeds. Eventually the entire plant breaks off at the base of the stem due to the force of wind and rolls over the ground, shedding the seeds all along the way. Such rolling plants are collectively known as tumble weeds.

Hairs : In cotton, hairs are the outgrowth from the seed coat and occur all along its surface.

Persistent styles : Clematis, Naravelia, Geranium etc. have persistent and feathery styles which help the fruit to be easily carried by wind.

Balloon like appendages : In plants like Cardiospermum and Nicandra fruits develop balloon like appendages which make the fruits light to be easily carried by wind.

Dispersal by water (Hydrochory)

Fruits and seeds, specialized for dispersal by water, generally develop some kind of floating devices and a protective covering which makes them water resistant. e.g., fibrous mesocarp in Coconut, spongy thalamus in Lotus.

Dispersal by animals (Zoochory)

Fruit and seeds dispersed by animals can be divided into following three categories on the basis of their adaptive features :

Hooked fruits and seeds : The surface of many fruits is covered with hooks (e.g., Xanthium, Urena), barbs (e.g., Andropogon), spines (e.g., Tribulus), bristles (e.g., Pupalia), or stiff hairs (e.g., Aristida), by means of which they adhere to the body of animals or clothes of human beings and they are carried unwarily from one place to another.  

Sticky fruits and seeds : Some fruits like those of Boerhaavia, Cleome, and Plumbago have sticky glands by which they adhere to the fur of grazing animals and are thus dispersed. Seeds of Viscum (mistletoe), Loranthus, etc. have a viscid layer which adhere to the beak of the bird which eat them.

Edible fruits : Human beings, birds, squirrels, bats, etc. are of great help in the dispersal of edible fruits from one place to another.

Dispersal by explosive or Spring like mechanism (Autochory)

A less common method of seed dispersal is by means of explosive fruits. Such fruits open with force and scatter the seeds in all directions. e.g., Balsam fruit (Impatiers), Oxalis, night jasmine (Nyctanthus), castor (Ricinus), camel&aposs foot climber (Bauhinia vahlii).