Microbiology is one of the Biology Topics that involves the study of microorganisms, including bacteria, viruses, and fungi.
Development of Seed and Fruit Formation: Structure, Types, Difference, Significance
Fruit may be defined as a fertilized, mature, or ripened ovary. As a result of fertilization, the ovary becomes enlarges, and the style and other parts of a flower gradually dry and fall off. The enlarged mature and modified ovary contains the seed, which is called fruit. Sometimes the style or some other parts of the flower also remain along the fruit.
Types of Fruits
1. True fruit: A fruit that develops from only the ovary part of a flower and contains one or more viable seeds is called true fruit, e.g., Mango, Brinjal, etc.
2. False fruit (Pseudocarp): A fruit that develops from the ovary along with adjoining accessory floral parts (like sepal, petal, thalamus, etc) and contains one or more viable seeds is called false fruit, e.g., Apple, Fig, Dillenia, etc.
3. Parthenocarpic fruit: A fruit that develops from an unfertilized ovary is called parthenocarpic fruit, e.g., Banana, Grapes etc. This phenomenon of the development of parthenocarpic fruit is called parthenocarpy. Parthenocarpic fruits are either seedless or contain non-viable seeds. The application of synthetic growth substances by spraying or by injection causes parthenocarpic development.
Differences between True Fruit and False Fruit:
|True Fruit||False Fruit|
|1. Only the ovary takes part in the formation of fruit.||1. Ovary and other floral parts take part in the formation of fruit.|
|2. Wall of a true fruit is called a pericarp which is usually edible.||2. Edible parts may be calyx, thalamus, or bracts.|
|3. Ovary wall remains open.||3. Ovary is covered with calyx, thalamus, or bracts.|
|4. Examples: Mango, Brinjal, etc.||4. Examples: Apple etc.|
Structure of a Typical Fruit
A typical fruit consists of two main parts Pericarp and Seed.
It is the wall of the fruit; it may be thin and dry or may be thick and fleshy. A thick and fleshy pericarp is differentiated into three distinct regions:
- Epicarp: It is the outermost thin region that forms the skin of the fruit. In mango, it is green when young and becomes coloured when ripened.
- Mesocarp: It is the middle region which may be thin membranous (orange), hard, and stony as in palms. In mango, the mesocarp is fleshy, juicy, and edible.
- Endocarp: Innermost region which is generally hard and woody that covers the seed.
A fruit may bear one or more than one seed. Seed-coat may be closely adhered to the endocarp or may be easily separated.
Differences between Normal and Parthenocarpic Fruit:
|Normal Fruit||Parthenocarpic Fruit|
|1. Fruit is developed after fertilization.||1. Fruit is developed without fertilization.|
|2. Seeds are formed in this type of fruit.||2. Seeds are not formed, these are called seedless fruit.|
|3. Seeds germinate to form young plants.||3. Seeds are absent, so young plants are not formed.|
In botany, a fruit is part of a flowering plant that is derived from specific tissues of the flower, from one or more ovaries, and in some cases accessory tissues. Fruits are the means by which these plants disseminate seeds. Many of them that bear edible fruits, in particular, have propagated with the movements of humans and animals in a symbiotic relationship as a means for seed dispersal and nutrition, respectively. In fact, humans and many animals have become dependent on fruits as a source of food. Fruits account for a substantial fraction of the world’s agricultural output, and some (such as the apple and the pomegranate) have acquired extensive cultural and symbolic meaning.
The first stimulus for fruit development comes from pollination while the second stimulus is received from developing seeds. The tissue of the ovary wall is induced to grow and form fruit. The wall of the ovary gives rise to the wall of the fruits, called the pericarp. The mature pericarp may be fleshy or dry. Accordingly, the fruits may be fleshy (e.g., mango, guava, orange) or dry (e.g., groundnut, mustard). In some cases, the thalamus grows along with the ovary wall to form part of the fruit. Such fruits are called false fruits (e.g., apples, strawberries). In the apple, the fleshy thalamus surrounds the pericarp and centrally placed seeds. A fruit that develops from only the ovary part of the flower, is called true fruit, e.g., tomato, grape, mango, etc. In a few species, seedless fruits develop without fertilization. Such fruits are called parthenocarpic fruits, e.g., bananas. Parthenocarpic fruits may also be induced by growth hormones.
Inside the ovary/ovaries are one or more ovules where the megagametophyte contains the egg cell. After double fertilization, these ovules become seeds. As the ovules into seeds, the ovary begins to ripen and the ovary wall, the pericarp, may become fleshy (as in berries or drupes), or form a hard outer covering (as in nuts). In some multi-seeded fruits, the extent to which the flesh develops is proportional to the number of fertilized ovules. The pericarp is often differentiated into two or three distinct layers called the exocarp (outer layer, also called epicarp), mesocarp (middle layer), and endocarp (inner layer).
In some fruits, especially simple fruits derived from an inferior ovary, other parts of the flower (such as the floral tube, including the petals, sepals, and stamens), fuse with the ovary and ripen with it. In other cases, the sepals, petals, stamens, and style of the flower fall off. When such other floral parts are a significant part of the fruit, it is called an accessory fruit. Since other parts of the flower may contribute to the structure of the fruit, it is important to study flower structure to understand how a particular fruit forms.
In angiosperms after fertilization, the ovary develops into a fruit and the ovules develop into seeds. A mature seed contains a fully developed embryo within it. During germination, the embryo grows into a seedling.
A fertilized mature integument ovule that gives rise to a new plant is called a seed.
Types of Seeds
On the basis of the number of cotyledons
- Monocotyledonous seed: Seeds with one cotyledon are called monocotyledonous seeds, e.g., Maize, Wheat, Rice, etc.
- Dicotyledonous seed: Seeds with two cotyledons are called dicotyledonous seeds, e.g., peas, Gram, Mango, etc.
- Polycotyledonous seed: Seeds with more than two cotyledons are called polycotyledonous seeds, e.g., Pine. (Gymnospermic seeds)
On the basis of the presence or absence of endosperm
- Endospermic or albuminous seed: Seeds with endosperm are called endospermic or albuminous seeds, e.g., rice, wheat, castor, etc. Most of the monocotyledonous seeds are albuminous.
- Non-endospermic or ex-albuminous seed: Seeds without endosperm are called non-endospermic or exalbuminous seeds, e.g., pea, gram, arum, etc. Most of the dicotyledonous seeds are non-albumin- ous. Monocot, but non-endospermic seed, Orchid seed (Vanda roxburghii) Dicot, but an endospermic seed, Castor seed (Ricinus communis).
Development of Seeds
A seed is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization within the mother plant. The formation of the seed completes the process of reproduction in seed plants (starting with the development of flowers and pollination), with the embryo developed from the zygote and the seed coat from the integuments of the ovule. All seeds are of different size, shape, and colour.
Seeds have been an important development in the reproduction and spread of flowering plants, relative to more primitive plants such as mosses, ferns, and liverworts, which do not have seeds and use other means to propagate themselves. This can be seen by the success of seed plants (both gymnosperms and angiosperms) in dominating biological niches on land, from forests to grasslands both in hot and cold climates.
The term “seed” also has a general meaning that antedates the above—anything that can be sown, e.g., “seed” potatoes, “seeds” of corn or sunflower “seeds”. In the case of sunflower and corn “seeds”, what is sown, is the seed enclosed in a shell or husk, whereas the potato is a tuber.
As a result of stimulus from fertilization a number of changes occur in the tissue outside the embryo sac leading to the formation of seed. Integuments of the ovule form a seed coat. The outer integument forms the testa and the inner integument forms the tegmen. In some cases like litchi, a sort of third integument or aril is present, which forms an additional covering of seed. Some seeds like castor (Ricinus communis) have a spongy outgrowth near the micropyle which is known as a caruncle, it absorbs water during seed germination.
Funiculus forms the stalk of the seed. Ultimately stalk withers and leaves a minute scar called hilum. Endosperm provides nourishment to the growing embryo. With the growth of the embryo, the central part of the endosperm is eaten up. In some seeds, the endosperm persists in the seed as a storage tissue. Such seeds are called endospermic or albuminous, e.g., maize, wheat, barley, coconut, and castor. In others, the endosperm is completely eaten up by the growing embryo. Such seeds are non-endospermic or exalbuminous, e.g., pea, gram, bean, etc. In some seeds remains of nucellus persist. The residual nucellus which persists in the seed is called perisperm, e.g., coffee, castor, cardamom, and Nymphaea. The micropyle of the ovule is changed in the micropyle of the seed. Through this pore oxygen and water enter the seed at the time of germination.
The embryo is composed of different parts; the epicotyl will grow into the shoot, the radicle grows into the primary root, the hypocotyl connects the epicotyl and the radicle, and the cotyledons form the seed leaves. Monocotyledonous plants have other structures; instead of the hypocotyl and epicotyl, it has a coleoptile that forms the first leaf and connects to the coleorhiza which connects to the primary root. Adventitious roots form from the sides. The seeds of corn are constructed with these structures; pericarp, and scutellum (single large cotyledon) that absorbs nutrients from the endosperm, endosperm, plumule, radicle,, coleoptile, and coleorhiza—these last two structures are sheath-like and enclose the plumule and radicle, acting as a protective covering. The testae or seed coats of both monocots and dicots are often marked with patterns and textured markings or have wings or tufts of hair.
Differences between Monocot and Dicot Seeds:
|Monocot Seed||Dicot Seed|
|1. Seed bears a single cotyledon.||1. Seed bears two cotyledons.|
|2. Pericarp and seed coat remain fused.||2. Pericarp and seed coat remain free.|
|3. Seed is generally endospermic.||3. Seed is generally non-endospermic.|
|4. Cotyledon is thin and inconspicuous.||4. Cotyledon is thick and conspicuous.|
|5. Embryo occupies one side of the seed.||5. Embryo occupies the central part of the seed.|
|6. Coleoptile and coleorhiza are present.||6. Coleoptile and coleorhiza are absent.|
Difference between Integument and Testa:
|1. Integument is the covering of the ovule.||1. Testa is the outer covering of the seed.|
|2. Here, sclereids are absent.||2. Here, the cells are rich in sclereids.|
|3. It is a pre-fertilized structural form.||3. It is a post-fertilized structural form.|
Difference between Perisperm and Pericarp:
|1. It is a dry part of the seed.||1. It is a dry or fleshy part of the seed.|
|2. It is the non-functional and unused persistent nucellus in the seed.||2. It is covering part of the fruit and helps in the dispersal and nutrition.|
Role of Seed
Seeds serve functions for the plants that produce them. Key among these functions is the nourishment of the embryo, dispersal to a new location and dormancy during unfavorable conditions. Seeds fundamentally are means of reproduction and most seeds are the product of sexual reproduction which produces a remixing of genetic material and phenotype variability on which natural selection acts.
Significance of Seed
- Dependable method: Unlike cryptogams, pollination and fertilization of seed plants are free from the requirement of water. Seed formation is, therefore, more dependable.
- An evolved structure: The structure of the seed is highly evolved as it provides protection to the embryo.
- Perennation: Thick protective seed coat and dormant embryo of the seed are most suitable for perennation.
- Reserve food: Seeds have reserve food for nourishing the young seedlings till they become nutritionally independent.
- Dispersal: Seeds possess a suitable mechanism for their own dispersal so that they may be spread to distant areas for colonization.
- Variations: Seeds carry a number of variations as they are the products of sexual reproduction. Variations are important for adaptation to diverse environmental conditions.
- Storage: Seeds can be stored for later use. This is helpful for the supply of food throughout the year and to overcome drought and famine conditions.
- Agriculture: Seed is the basis of agriculture. Agriculture originated when humans learned to eat, store and sow seeds. Agriculture proved to be the turning point for the evolution of human civilization, industrialization, science, and technology.
Significance of Fruit
- Fruit protects immature seeds from unfavourable environmental conditions. The seeds remain enclosed in the fruit until they are ready to germinate.
- The green colour of young fruit is also a measure for the protection of seeds because they remain hidden within the foliage.
- The immature fruits offer chemical defense against the animal because they contain unpalatable and repelling substances like astringents, tannins, sour acids, and bitter alkaloids.
- Mature fruits acquire bright colours to attract the seed-dispersing animals.
- Certain fruits on maturity burst with great pressure so that the seeds are dispersed to a distance.
- Fruits are in use since prehistoric times as the main food of man. Even today fruits comprise the main part of the human diet.
Special Modes of Reproduction in Flowering Plants
Apomixis is an asexual mode of reproduction in which new individuals are formed without the formation of gametes and their fusion.
Types of Apomixis in Flowering Plants
The following simple classification of types of apomixis in flowering plants:
In this type “the megaspore mother cell undergoes the usual meiotic divisions and a haploid embryo sac (megagametophyte) is formed. The new embryo may then arise either from the egg (haploid parthenogenesis) or from some other cell of the gametophyte (haploid apogamy).” The haploid plants have half as many chromosomes as the mother plants and “the process is not repeated from one generation to another” (Which is why it is called non-recurrent).
Recurrent Apomixis/Gametophytic Apomixis:
In this type, the megagametophyte has the same number of chromosomes as the mother plant because meiosis was not completed. It generally arises either from an archesporial cell or from some other part of the nucellus.
Adventive Embryony/Sporophytic Apomixis/Sporophytic Budding/Nuceliar Embryony:
There may be a megagametophyte in the ovule, but the embryos do not- arise from the cells of the gametophyte; they arise from cells of the nucellus or the integument. Adventive embryonic is important in several species of Citrus, Garcinia, Euphorbia dulcis, Mangifera indica, etc.
In this type, the flowers are replaced by bulbils or other vegetative propagules which frequently germinate while still on the plant. Vegetative apomixis is important in Allium, Fragaria, Agave, and some grasses, among others. The term ‘Apomixis’ was introduced by Wrinkier (1908). Apomixis occurs by the following methods:
- Agamospermy: A mode of apomixis in which seeds are formed but are asexual in nature as the embryo develops directly without gametic fusion. It is commonly formed from a diploid nucellar cell (apospory) or megaspore mother cell.
- Parthenogenesis: It is the development of a new individual from a single gamete without fusion with another gamete. In lower plants, both kinds of gametes may undergo parthenogenesis but in higher plants usually the female gamete develops. Depending upon the ploidy of the gametes parthenogenesis may be diploid or haploid. In haploid parthenogenesis, the embryo sac and its egg are haploid. The egg grows into the haploid embryo. Such embryos are formed in lower plants. In diploid parthenogenesis, the embryo sac and its egg are diploid. It undergoes parthenogenesis and forms the diploid embryo. It is generally accompanied by failure of meiosis during megasporogenesis as well as the direct formation of embryo sac from a nucellar cell, e.g., Rubus, Poa, apple, etc.
- Apogamy: It is the formation of a sporophyte or embryo directly from cells of the gametophyte. In higher plants, only diploid apogamy is successful, that is, gametophytic tissue forming the sporophyte is diploid. In lower plants, apogamy is equally successful.
Incidence of Apomixis in Flowering Plants
Apomixis occurs in at least 33 families of flowering plants and has evolved multiple times from sexual relatives) Apomictic species of individual plants often have a single origin and are usually polyploid. In plants with both apomictic and meiotic embryology, the proportion of the different types can differ at different times of the year, and photoperiod can also change the proportion. It appears unlikely that there are any truly completely apomictic plants, as low rates of sexual reproduction have been found in several species that were previously thought to be entirely apomictic.
The genetic control of apomixis can involve a single genetic change that affects all the major developmental components, formation of the megagametophyte, parthenogenesis of the egg cell, and endosperm development. However, the timing of the various developmental process is critical to the successful development of an apomictic seed and the timing can be affected by multiple genetic factors.
Advantages and Disadvantages of Apomictic Crop
- Advantage: In hybrid progeny no segregation of characters. Year after year apomictic hybrid can be used to grow crops.
- Disadvantage: Deleterious genetic mutation cannot control. Lack of variations reduces genetic diversity from parents to offspring.
In botany and horticulture, Parthenocarpy (literally meaning ‘virgin fruit’) is the natural or artificially induced production of fruit without fertilization of ovules. The fruit is therefore seedless. Stenospermocarpy may also produce apparently seedless fruit, but the seeds are actually aborted while still small. Parthenocarpy (or stenospermocarpy) occasionally occurs as a mutation in nature, but if it affects every flower, then the plant can no longer sexually reproduce but might be able to propagate by vegetative means.
Production and development of seedless fruits are called parthenocarpy, e.g., pineapple, banana, apple, pear, etc. In some cases, the stimulus of pollination is required. The phenomenon is called stimulative parthenocarpy. In others, parthenocarpy occurs in unpollinated flowers. It is vegetative parthenocarpy. Parthenocarpy is of three types-genetic, environmental, and chemically induced.
- Genetic Parthenocarpy: Parthenocarpy is due to genetic alteration caused by mutation or hybridization. It is also called natural parthenocarpy, e.g., banana, pineapple, grape, pear, etc.
- Environmental Parthenocarpy: Low temperature, frost, and fog have been known to induce parthenocarpy in a number of plants, e.g., pear, tomato, and capsicum.
- Chemically induced Parthenocarpy: Spray or paste of auxins and gibberellins in low concentrations of 10-6 to 10-7 M has been found to induce parthenocarpy in several plants, e.g., tomato, cucurbits, Citrus, strawberry, blackberry, etc.
Importance of Parthenocarpic Fruits
- They do not contain irritant seeds which have to be removed before eating fruits.
- Fruits can be developed inside greenhouses where pollinators are not available.
- Quicker food processing.
- Seedlessness is seen as a desirable trait in edible fruit with hard seeds such as pineapple, banana, orange, and grapefruit. Parthenocarpy is also desirable in fruit crops that may be difficult to pollinate or fertilize, such as tomatoes and summer squash.
- In dioecious species, such as Persimmon, parthenocarpy increases fruit production because staminate trees do not need to be planted for pollen. Parthenocarpy is undesirable in nut crops, such as pistachio, where the seed is the edible part.
- Horticulturists have selected and propagated parthenocarpic cultivars of many plants, including fig, cactus (Opuntia), pear, breadfruit, and eggplant. Some plants, such as pineapple, produce seedless fruits when a single cultivar is grown because they are self-infertile.
- Some cucumbers produce seedless fruit if pollinators are excluded. Strange as it seems, seedless watermelon plants are grown from seeds. The seeds are produced by crossing a diploid parent with a tetraploid parent to produce triploid seeds.
Polyembryony is the phenomenon of two or more embryos developing from a single fertilized egg (in humans; identical twins). Polyembryony occurs regularly in many plants and animals. The nine-banded armadillo for instance, usually gives birth to four identical young ones. Polyembryony is best known among families of Encyrtidae, Dryinidae, Platygasteridae, and Braconidae. The term is also used in botany to describe the phenomenon of two seedlings emerging from one seed.
In plants, polyembryony often gives rise to the enigma of a single offspring. The mechanism underlying this phenomenon is programmed cell death (PCD) which removes all but one embryo. The phenomenon of the development of more than one embryo in the same seed is called polyembryony. It was first discovered by Leeuwenhoek (1719) in Citrus. Polyembryony is of three types – simple, cleavage, and adventive.
- Simple Polyembryony: Polyembryony occurs due to the fertilization of more than one egg. The condition develops when an embryo sac contains more than one egg cell or the ovule possesses more than one embryo sac, e.g., Brassica, Casuarina, Poa, Citrus, etc.
- Cleavage Polyembryony: Polyembryony is caused by the splitting of the proembryo into two or more parts and branching of the proembryo. It is common in conifers (e.g., Pinus), and also observed in angiosperms, e.g., Orchids, Nymphaea, and Nicotiana.
- Adventive Polyembryony: When additional embryos are formed from different parts of the ovule like synergids, antipodal cells, nucelius, integuments, etc., e.g., Citrus, Opuntia, Mangifera, etc.
If extra embryos develop from the same embryo sac, the polyembryony is called true polyembryony. If the extra embryos are formed elsewhere, it is called false polyembryony.
A more striking example of the use of polyembryony as a competitive reproductive tool is found in the parasitoid Hymenoptera of the family Encyrtidae. The progeny of the splitting embryo develop into at least two forms, those that will develop into adults and those that become a type of soldier, called precocious larvae. These latter larvae patrol the host and kill any other parasitoids they find with the exception of their siblings, usually sisters.