Fruits & seeds

Fruits & seeds

1. FRUITS AND SEEDS

2. FRUIT

3. Fruit is simply the fertilised and ripened ovary, together with any floral organ that may be attached to it. Fruits, developed entirely from the ovary, is called true fruits . In some cases, other floral parts may also be incorporated with fruits, such fruits are called pseudocarps or false fruits (e.g., apple, fig fruit). The pulpy part of such fruit is the flesty receptacle.

4. Development of fruit Fruit development starts soon after pollination. In fact, pollination stimulates the growth and development of ovary and also prevents its abscission. The auxins , secreted by the pollen grains and the carpellary tissues, are believed to initiate the growth of ovary. Further growth is stimulated by the auxins , gibberellins and cytokinins , secreted by seeds. Thus, seeds play a key role in fruit development.

5. The major changes, taking place during fruit development, are the following: (i) Ovary enlarges by repeated cell division and cell enlargement, induced by auxins , gibberellins and cytokinins . (ii) Succulent parenchyma cells develop within the ovary. (iii) Cells get loaded with vegetable acids, sugars and flavouring substances. (iv) Dissolution of the existing walls in some cells, and formation of false septa in others. This alters the nature and appearance of the ovary. (v) Transformation of the ovary wall to the fruit wall, called pericarp .

6. No further growth occurs in a mature fruit. So, it soon undergoes ripening. During this, certain biochemical changes occur giving the fruit its colour , taste and flavour . Such changes include the conversion of starch to sugar, decrease in acid concentration, production of esters, breakdown of chlorophyll, transformation of chloroplasts to chromoplasts , etc. It has been shown that ripening of the fruit is hastened by ethylene, formed in it in small amounts. So, ethylene treatment can cause the artificial ripening of fruits.

7. Significance of fruit formation Fruit formation in angiosperms is biologically significant, since fruits serve the following functions: ( i ) Seed protection - Fruits cover and protect immature seeds against animals and hostile environmental conditions (until the seeds are able to withstand unfavourable conditions or ready to germinate). (ii) Seed dispersal - Fruits help the dispersal of seeds by wind, water and animals, colouration or taste of fruits attracts seed-dispersing animals. (iii) chemical defence - Immature fruits offer chemical defence against animals. Since they contain unpalatable and repellent chemicals, such as astringents, bitter alkaloids, sour acids, tannins, etc. (these chemicals may disappear on ripening)

8. Different types of fruits Fruits are produced only by angiosperms. They are of three major groups, simple , aggregate and multiple . Simple fruit develops from a single monocarpellary or syncarpous pistil, whereas aggregate fruit develops from an apo carpous pistil. In aggregate fruit, each free carpel of the pistil develops to a small fruit. These small fruits may remain free from each other in the form of a cluster (e.g., Polyalthia ) or they may fuse to form a single fruit (e.g., Annona ): Multiple or compound fruit develops from a complete inflorescence. In this case, the ovary of each flower develops to a fruit and all the fruits thus formed then fuse together to form a multiple fruit (e.g., jack fruit, pine apple ).

9. Simple fruits are of two kinds, fleshy and dry . In fleshy fruits, the fruit wall or pericarp is fleshy and juicy, whereas in dry fruits, the pericarp is hard and dry. Dry fruits are of three kinds, dehiscent , indehiscent and schizocarpous . In dry dehiscent fruits, after a certain stage of development, the dry pericarp splits or breaks open and liberate the seeds. In dry indehiscent fruits, the pericarp does not dehisce at maturity. Schizocarpous fruits are intermediate between dehiscent and indehiscent types.

10. Dry dehiscent fruits developed from monocarpellary pistils The dry dehiscent fruits, developed from monocarpellary pistils are of different types. The commonest ones are legume or pod follicle siliqua capsule

11. ( i ) Legume or pod This is a dry, dehiscent fruit, developed from monocarpellary, superior and unilocular ovary that has marginal placentation and many ovules. Ripe fruit has a row of seeds on the inner side of a ventral suture. It dehisces by dorsal and ventral longitudinal sutures producing two valves, each bearing ovules Dehiscence is due to the differential drying of the carpel wall and it often results in the explosive release of seeds, or in the twisting of valves to dislodge the seeds. Legumes are characteristic of Leguminosae -e.g.. Pea pods.

12. Legume or pod

13. (ii) Follicle This is a type of dry, dehiscent fruit, developed from monocarpellary, superior and unilocular ovary with marginal placentation and many seeds. Ripe fruit dehisces by one suture, usually the ventral one. e.g., Callotropis , Michelia .

14. Follicle

15. (iii) Siliqua ( silique ) This is a dry, dehiscent fruit, developed from bicarpellary , syncarpous and superior ovary with parietal placentation . At maturity, the pericarp dehisces into two valves. Siliqua is characteristic of cruciferae .

16. Siliqua ( silique

17. iv) Capsule A dry, dehiscent fruit, developed from polycarpellary, syncarpous and superior ovary (sometimes inferior ovary also) which is multilocular with many seeds in each locule . e.g., Catheranthus .

18. Dry dehiscent fruits developed from syncarpous pistils Dry, dehiscent fruits derived from syncarpous pistils are commonly known as capsules . These capsules are variously named according to their mode of dehiscence. The common capsules are loculicidal , septicidal , septifragal and porous capsules.

19. ( i ) Loculicidal capsule This is a dry, dehiscent fruit capsule in which fruit wall bursts in the middle of the each locule into as many valves as there are carpels . Sometimes, placenta also splits. e.g., Gossypium (cotton), Abelmoschus esculentus (lady's finger).

20. (ii) Septicidal capsule A dry, dehiscent fruit capsule which dehisces by longitudinal splitting through the middle septum. Seeds are not exposed during dehiscence and so they are released as loculicidal capsules. e.g., Aristolochia .

21. (iii) Septifragal capsule A dry dehiscent fruit capsule which dehisces by breaking the outer wall away from the walls of the loculi . In loculicidal and septicidal capsules, the pericarp will continue to remain attached to the septum even after dehiscence. But, in septifragal capsules, the pericarp breaks away completely from the septa. The first dehiscence may be loculicidal or septicidal . In the former case, the capsule is called loculicidally septifragal (e.g., Lagerstroemia ), and in the latter septicidally septifragal (e.g.. Datura )

22.  (iv) Porous capsule A dry, dehiscent fruit capsule with numerous small holes on its top. Through these holes seeds escape – e.g., poppy .

23. Dry indehiscent fruits These are the single - seeded fruits which do not burst or dehisce to expose or release seeds. In them, pericarp ruptures during seed germination. The common types of dry indehiscent fruits are achene utricle caryopsis cypsela samara nut.

24. ( i ) Achene This is a small, simple, single-seeded, unilocular , indehiscent, dry fruit, developed from monocarpellary superior ovary. Its pericarp is dry, membranous and free from seed-coat: e.g., Mirablilis (4'o' clock plant).

25. Achene

26. (ii) Utricle This is an indehiscent, bladder-like, dry fruit formed in some members of Amaranthaceae and Chenopodiaceae . It is a modified form of achene . Here the small seed occupies only a portion of the fruit: e.g., Amaranthus .

27. Utricle

28. (iii) Caryopsis An indehiscent, dry, single-seeded fruit, developed from monocarpellary, superior, unilocular ovary. Its pericarp fuses with the seed-coat. Usually, the grain is covered by persistent bract and bracteoles. Caryopsis is characteristic of Graminae . e.g., wheat, maize .

29. Caryopsis

30. (iv) Cypsela Is a type of single- seeded,unilocular , indehiscent dry fruit, formed from bicamellary , syncarpous , inferior ovary. Its pericarp is separate from the seed-coat. Cypsela is characteristic of compositate , where fruit is surrounded by hairs ( pappus ), derived from calyx. Traditionally, cypsela is considered as a pseudocarp . e.g., Sunflower .

31. (v) Samara Is a type of single-seeded, dry, indehiscent fruit, with a membranous Wing-like extension of the pericarp for the wind dispersal of seeds. It is simply a winged achene . e.g.. Pterocarpus , Holoprelia .

32. (vi) Nut Nut is a hard, usually single-seeded indehiscent fruit, derived from a syncarpous ovary. During its development, all seeds, excepting one, get aborted. Its pericarp is usually lignified and often partially or completely surrounded by a cupule . e.g., Cashew nut .

33. Schizocarpic fruits Sohizocarp is intermediate between the indehiscent and dehiscent types of dry fruits. (e.g.. Geranium, Acacia, Coriander ). It is formed from two or more one- seeded carpels which divide into single-seeded units at maturity. These single-seeded units include achenes , follicles , nutlets , samaras , mericarp , cremocarp , lomentum , carcerulus etc.

34. ( i ) Mericarp Mericarp is a single-seeded portion of a fruit which splits at maturity. It may be dehiscent or indehiscent.

35. (ii) Cremocarp Cremocarp is a type of bilocular , two-seeded schizocarp, developed from bicarpellary , syncarpous , inferior ovary. At maturity, it splits into two single-seeded and unilocular units. Mericarps hang down from forked carpophore . They are found among the members of Umbelliferae .

36. (iii) Lomentum Lomentum is an elongated, dry schizocarp, developing from monocarpellary, superior ovary that has one or more seeds. At maturity, it develops transeverse constrictions and finally splits through these constrictions to form one-seeded portions. e.g., groundnut . 

37. (iv) Carcerulus It is a dry schizocarp that develops from bicarpellary , syncarpous , superior ovary. At maturity, it splits into several single-seeded segments. Carcerulus is characteristic of Lamiaceae .

38. Fleshy fruits These are the fruits with succulent and fleshy pericarp . There are several kinds of fleshy fruits. The commonest ones among them are drupe , berry , pome , pepo , hesperidium , etc. 

39. ( i ) Drupe Drupe is a mono or multilocular , single-seeded, indehiscent or fibrous fleshy fruit whose pericarp is well differentiated into thin epicarp , fleshy or fibrous mesocarp , and hard endocarp . It is formed from a monocarpellary or polycarpellary, syncarpous , superior ovary. e.g., mango, plum, peach, coconut .

40. mango plum coconut peach

41. (ii) Berry ( Bacca ) Berry is a many-seeded, indehiscent pulpy fruit, developed from mono or polycarpellary, superior or inferior ovary with axile or parietal placentation . In it, mesocarp alone is well developed and it forms the succulent pulp in which seeds remain embedded. Epicarp form a thin "skin" of the fruit. tomato, banana, grape, sapota , Endocarp may be very thin, or even absent. e.g., brinjal .

42. (iii) Pome Pome is, in fact, a highly specialized false fruit because its succulent edible part is developed not from the ovary, but from the fleshy receptacle of a syncarpous , inferior ovary. The receptacle encloses the true fntits in its core. The 'skin' of the fruit is, therefore, not the epicarp and the fleshy portion is not the mesocarp . pome is typical of Rosaceae . The common example is apple.

43. (iv) Pepo Pepo is a many-seeded, fleshy and often large-sized fruit, formed from an inferior, syncarpous ovary. It is, in fact, a specialized type of berry. The outer portion of its epicarp forms leathery covering. Pepo is characteristic of Cucurbitaceae . e.g., melons, cucumbers.

44. (v) Hesperidium Hesperidium is a multi seeded, fleshy fruit, formed from a multi camellary , syncarpous , superior ovary with axile placentation . Its pericarp is differentiated into epicarp , mesocarp and endocarp. Epicarp is leathery, with oil glands. Mesocarp is white and fibrous. Endocarp has numerous unicellular and fluid - filled trichomes which project into the locules and form characteristic segments. Endocarp is the edible part. e.g.. lemon, orange .

45. Aggregate fruits Aggregate fruit is a 'cluster fruit', formed from an apocarpous pistil Of a single flower. So, it consists of a number of individual fruits. It may be an aggregate Of achenes . berries. follicles or samaras. The term etaerio is mostly used to designate Aggregate fruit . During fruit formation, each individual carpel remains free and develops to a fruitlet . So, a flower gives rise to a cluster of small fruitlets .

46. In some cases, the fruitlets are free from each other so that a fruit represents a bunch of fruitlets (e.g., Polyalthia . Artabotrys ). But, in others, the carpels unite together to form a single massive fruit (e.g., Annona ). Each small elevated area on the fruit of Annona represent a carpel of immediately below it there will be single seed. An aggregate fruit may be dehiscent or indehiscent. In Michelia , the fruit is an aggregate of follicles, in strawberry, an aggregate of achenes , and in Artaborrys an aggregate of berries

47. Multiple fruits Multiple fruit is a massive, fleshy, compound fruit, formed from an entire inflorescence. It is formed by the fusion of the ovaries and other floral parts (such as bracts, pedicels and receptacles) of entire flowers of the inflorescence. Sorosis , syconus , coenocarpium , etc. are the different types of multiple fruits.

48. Sorosis is a composite fruit that develops from a spike, spadix , or catkin type of inflorescence — e.g., Artocarpus , Morus .

49. Jack fruit ( Sorosis ) Jack fruit is an example of sorosis . The inflorescence in jack is unisexual. The female spike develops into the fruit. If the fruit is cut longitudinally into two halves, there is a long central axis, which is the floral axis or peduncle. The sweet edible flakes represent the perianth of the flowers which have become thick and succulent. Inside the edible portion, there is a membranous bag containing a big seed. The bag represents the pericarp . Between the edible flakes, there are numerous flat, elongated, whitish structures which are not edible. These are the sterile or unfertilized flowers. The hard projecting portions seen outside represent external sheaths, through the centre of which pass the styles of the flowers. 

50. Syconus is the multiple fruit that develops from a hypanthodium type of inflorescence — e.g., Ficus . In this case, receptacle form a hollow succulent cavity, opening out by a small apical pore.

51. Coenocarpium is a multiple fruit formed by the fusion of the ovaries, floral parts and receptacles of many flowers. It has a fleshy axis — e.g., Ananas

52. Pine apple Pine apple is an example of Pine-apple coenocarpium . The inflorescence axis becomes fleshy and all the flowers fuse together. The inflorescence axis continues to grow beyond the fruit and produces a few leaf-like bracts forming a crown on the fruit. On the surface of the fruit numerous polygonal areas are seen. Each of them represents a flower and the corresponding bract.

53. SEED

54. During seed formation, tertilized ovule undergoes a series Of development and it becomes the seed. The two integuments of the ovule become the seed coat. The outer integument is known as the testa , while the inner one, the tegmen . In many cases, the tegmen is completely fused with the testa . The fertilised egg develops into an embryo. Each embryo has three distinct portions, namely ( i ) a rudimentary root portion, called the radicle that faces the micropyle , (ii) a shoot portion known as the plumule , and (iii) one or two flat, expanded structures called the cotyledons in between the radicle and the plumule . These cotyledons are almost leaf-like. Plants, such as bean, pea and tamarind, have two cotyledons in their seeds and so they are known dicotyledons or dicotyledonous plants. But, in paddy, maize, wheat, coconut, there is only a single cotyledon inside the seed and so these plants are known as monocotyledons or monocotyledonous plants.

55. The food material required for the of the embryo is not always stored in the endosperm. Sometimes, the food material is stored up in cotyledons in the form of starch, protein, or oil. The cotyledons absorb all the nourishment, and become thick and fleshy. Hence, there is no endosperm inside such seeds and they are called non- endospermous seeds , e.g., bean, pea . seeds having endosperm are called endospermous or albuminous seeds , - e.g., castor, wheat, date . All seeds have endosperm at the beginning, but it may disappear as the seed matures.

56. The chemical composition of the endosperm is not the same in all plants. In castor seed, plenty of oil is stored up in the endosperm and so the endosperm is oily as ill many oil seeds. In paddy, wheat and other food grains, endosperm is rich in starch' Such an endosperm is called mealy or farinaceous . In coconut, kernel is tile is utilized by the developing endosperm. But, in certain seeds like water-lily and pepper, even after the seeds are fully formed, a small amount of nucellus is left behind unused. This remaining portion of the nucellus is known as the perisperm and seeds having it are described as perispermous .

57. Thus, based on storage centres of reserve food, three main types of seeds can be recognized as follows: ( i ) Endospermous or albuminous seeds — e.g., castor, tamarind . (ii) Non- endospermous or exalbuminous seeds — e.g., peas, beans, grams . (iii) Endospermic and perispermic seeds — e.g., black pepper, water- lilly .

58. A. Exalbuminous dicot seed (e.g., Pea) Pea seed is an exalbuminous dicot seed. It attaches to the fruit wall by a short stalk, called funiculus . At maturity the funiculus detaches from the seed, leaving a scar on the seed surface, called hilum . Close to the hilum is the micropyle . It serves for the absorption of water during germination. Seed has a tough seed-coat, formed by the fusion of testa and tegmen . Testa is the outer coat, and tegmen the inner one. These are developed from the integuments of the ovule. Seed coat has a longitudinal ridge, called raphe .

59. The whole of the seed, enclosed within the seed coat, is called kernel . There are two fleshy cotyledons, attached to the embryonal axis. In the absence of endösperm the fleshy cotyledons store food materials for the embryo. The tip of the embryonal axis is called plumule or shoot primordium , and its base is called radicle or root primordium . The portion of the embryonal axis, in between the plumule and the cotyledonary node, is called epicotyl (above cotyledons). The portion between the radicle and the cotyledonary node is called hypocotyl (below cotyledons).

60. B. Albuminous monocot seed (e.g. rice) Most monocot seeds are albuminous . The grain of paddy is a typical example. Its outer covering or husk has two halves, namely, lemma and palea . Inside the husk is a thin and brown coat, called bran . It is formed by the fusion of pericarp and seed coat. Micropyle and hilum are not clearly seen due to the presence of pericarp . The large starchy endosperm forms the bulk of the grain. It is surrounded by a proteina - ceous sheath, called aleurone layer The embryo is seen basally on one side of the endosperm. The embryonal axis is differentiated into plumule and radicle . Plumule is surrounded by a protective Bran sheath. called coleoptile ( plumule sheath). and radicle by coleorhiza (root sheath).

61. In between the embryo and the endosperm is a single cotyledon. It is modified as a shield- like absorptive and secretory organ, called scutellum . It secretes enzymes to digest the endosperm during seed germination. Scutellum is separated from the endosperm by an epithelial layer . The part of the embryonal axis, lying in between the coleoptile and the cotyledonary node, is sometimes called mesocotyl .

62. Dispersal of fruits and seeds Plants produce large numbers of seeds. Far and wide dispersal of these seeds is essential for the distribution and establishment of plants over extensive geographical areas. Also, it enables the colonization of new areas, reduces overcrowding in a given area, and minimizes intraspecific and interspecific competitions for light, air, water. minerals. etc. Some plants disperse their seeds by the explosive bursting of their pods or capsules. In others, dispersal is carried out by external agencies, such as wind , water and animals .

63. Dispersal by wind ( anemochory or anemospory ) In order to facilitate easy dispersal by wind, fruits and seeds must be made light. This is accomplished by various specializations some of which are the following: ( i ) Thin and light fruits and seeds — e.g., Orchids . (ii) Hairy ( cosmose ) seeds, with tufts of delicate hairs - e.g., Bombax , calotropiS , Alstonia , Gossypium (iii)Winged fruits or seeds with lateral expansions. Winged fruits are seen in Calycopteris , and winged seeds in spathodea , Tecoma , Plumeria , etc. ( iV ) Persistent feathery styles — e.g., Narvelia , Clematis . All these specializations enable fruits and seeds to float in air and to parachute or drift with air movements.

64. Dispersal by water ( hydrochory or hydrospory ) This is common in plants growing along sea-shores and river banks. Fruits and seeds dispersed by water have invariably a thick outer coat for protection from damage. In many cases, the pericarp contains air spaces so that the fruits are rendered light and they can float easily and are carried to considerable distances by water currents. In coconut and Gerbera , the pericarp has a fibrous portion enclosing numerous air spaces. In Calophyllum , air is present between endocarp and seed, thus making the fruits light. In water-lily, seeds are provided with a spongy aril. In lotus, thalamus itself gets detached and floats in water, carrying the fruits.

65. Dispersal by animals ( zoochory or zoospory ) Quite a number of fruits and seeds are dispersed by mamals and birds. They are characterized by many features. In some cases, fruits are provided with spines and hooks. When animals brush against such plants the fruits cling to their bodies and are thus carried to various places. The fruits of Tribulus have sharp spines on them and they stick on to the feet of animals. Glandular hairs are found on many fruits. Their glands secrete a sticky juice which helps in dispersal. The fruits stick to the bodies of animals and are carried by them.

66. Some seeds are specially coloured in a deceptive manner. Birds mistake these seeds for insects and carry them away. Many fleshy, succulent fruits are edible. Birds and other animals eat the fleshy portion and drop the seeds in various places. Birds swallow fruits and the seeds and excreted out. The seeds, however, do not suffer any damage since they are protected by hard seed coats. In fact, the passage of the seeds through the alimentary canal assists in their germination. Fleshy fruits usually develop taste, colour and aroma only when they are ripe and the seeds are ready for dispersal.

67. Dispersal by the explosive bursting of fruits ( autochory ) In certain dry capsular fruits, the pericarp splits open explosively with a sudden jerk. As a result, the seeds are shot out to some distance. The furits of rubber tree, Clitoria , Abrus , etc. explode automatically, while those of Impatients , Oxalis, Viola , etc. explode and twist on being touched. In legumes and Clitoria , the two valves coil round spirally thus ejecting the seeds soon after dehiscence. Some plants keep their fruits under soil or water without dispersing. This is called carpotropy . Maturation of such seeds under soil is called geocarpy (e.g., Arachis ), and maturation under water is called hydrocarpy (e.g., Eichhornia , Linaria )