Do you know that all living organisms reproduce (both plants and animals)? Reproduction is a special biological process, by which new individuals of the same species are produced. It is one of the biological processes like nutrition, respiration and excretion.
MODES OF REPRODUCTION
Modes of reproduction in single cell organism: Let us examine how different organisms actually reproduce. The methods by which organisms reproduce depend upon the body shape and structure of organisms. Unicellular organisms, like amoeba and bacteria, split into two equal halves and each half develops into new ones. This method is called binary fission.
Modes of reproduction in multicellular organisms:
1. Vegetative propagation :-Fragmentation and Budding.
2. Asexual reproduction and Sexual reproduction:- Spores, Pollination and Fertilization.
Depending upon the body organization of multicellular organisms, there are various methods of reproduction.
Vegetative propagation:
It is the ability of plants to reproduce by bringing forth new plants from the existingvegetative structures without sexual
reproduction.
Fragmentation
In multicellular organisms with simple body organization, simple reproductive methods have been noticed.In Spirogyraalgae, the plant body breaks up into small fragments. Each fragment grows into a new individual.
Budding
In Hydra, a bud develops into an outgrowth due to repeated cell division at one specific site. These buds develop into tiny individuals and when fully matured, get detached from the parent body to become new independent individuals.
Similarly, buds produced in the notches along the leaf margin of Bryophyllum fall on the soil and grow into new plants (in Tamil katti pottal kutti podum).
Asexual reproduction:
In lower group of plants, asexual reproduction takes place by means of spores. The spores are covered by thick walls that protect them until they come into contact with another moist surface and begin to grow.
SEXUALREPRODUCTION IN PLANTS
Sexual reproduction is the process in which two gametes(male and female) are fused to produce offspring of their own kind.
A bull alone cannot produce calves. It needs a cow. A female sheep (ewe) alone
cannot produce lambs. It needs a male sheep (ram).
Both the sexes, male and female, are indispensable to produce offspring.You have already learnt that the flower is a reproductive organ of a flowering plant.To understand this further we need to first study the structure of a flower.
Parts of a typical flower
A floweris a modified shoot with a limited growth to carry out sexual reproduction.
The main whorlsof a complete flower are:
1. Calyx (Composed of sepals)
2. Corolla (Composed of petals)
3. Androecium
4. Gynoecium
Androecium is the male reproductive part of a flower and Gynoecium is the female reproductive part of a flower.
Androeciumis composed of stamens. Each stamen consists of a stalk called the filament and a small bag like structure called the anther at the tip. The pollen grains are produced in the anther within the pollen sacs.
Gynoeciumis the female part of the flower and is made of carpels. It has three parts :
1. Ovary
2. Style
3. Stigma
The ovary contains the ovules and each ovule carries within it an embryo sac, within which lies the egg cell or the female
gamete.
Pollination
How does sexual reproduction take place in flowering plants?
The sexual reproduction in flowering plants involves
1. Pollination
2. Fertilization
1. Pollination
The transfer of pollen grains from the anther to stigma of a flower is called pollination. Pollen grains are transferred mainly by wind, water, insects and animals. They are called pollinating agents.
Pollination is the first important event in the development of a fruit and seed. Pollination is followed by fertilization.
Types of Pollination
Pollination is of two types. They are:
1. Self pollination
2. Cross pollination
Self Pollination (Autogamy)
Self pollination is also known as autogamy. The transfer of pollen grains from the anther of a flower to the stigma
of the same flower or another flower of the same plant is known as self pollination.
Advantages of self pollination
1. Self pollination is certain in bisexual flowers.
2. Flowers do not depend on agents for pollination.
3. There is no wastage of pollen grains.
Disadvantages of self pollination
1. The seeds are less in number.
2. The endosperm is minute. Therefore, the seeds produce weak plants.
3. New varieties of plants cannot be produced, resulting in the degradation of the plant.
Cross Pollination (Allogamy)
The transfer of pollen grains of a flower to the stigma of another flower of a different plant of the same species is called cross pollination or allogamy.
Advantages of cross pollination
1. The seeds produced as a result of cross pollination, develop and germinate properly and grow into better plants, i.e.
cross pollination leads to the production of new varieties.
2. More viable seeds are produced.Agents of cross pollination How is it possible to transfer pollen grains from one flower to another?
In order to bring about cross pollination, it is necessary that the pollen should be carried from one flower to another of a different plant. This takes place through the agency of animals, insects, wind and water.
a) Pollination by animals (Zoophily)
b) Pollination by birds (Ornithophily)
c) Pollination by insects (Entamophily)
Entamophily
Insects like butterflies and honey bees are attracted to the bright petals of the flowers. These flowers are large in size and have a sweet smell. Some of these flowers produce nectar. This is the mostcommon of all types of pollination. This kind of pollination is called Entamophily. (Pollination by insects).
Anemophily (Pollination by wind)
The flowers pollinated by wind are mostly small in size and do not have any attractive colour, smell and nectar. They produce a large number of pollen grains to make up for the wastage of pollen in transit.
The pollen grains are non-sticky, dry, light, powdery and hence are easily carried by the wind.
Some pollen grains even have wings. Stigmas are large and protruding, even branched and feathery. e.g.maize.Flowers pollinated by wind are called Anemophilous, e.g. grass and pine.
Pollination by Water (Hydrophily)
Pollination by water is called hydrophily. It is observed in some aquatic plants like Vallisneria, Hydrilla, Zosteria.The flowers of these plants are not colourful and have no nectar.Pollen grains have mucilaginous covering to protect them from getting wet.
FERTILIZATION
Recall what you have studied about pollination.Pollination is the transfer of pollen grains from the anther to the stigma. Each pollen grain has protective walls calledexine and intine. The outer wall exine is thick and it has small pores called
germination pores. The inner wall is thin and elastic.
Germination of pollen grain
If a pollen grain falls on a suitable stigma, it starts germinating. A mature pollen consists of two cells. The larger one is vegetative cell and the smaller one is generative cell. The vegetative cell starts growing and emerges through the germination pore. It develops through the style as a long tube known as pollen tube. The generative cell gets into the tube and divides into two male gametes (sperms).
Process of fertilization
The pollen tube enters into the embryo sac through micropyle. At this time, the pollen tube bursts open, gametes are released from the pollen tube and enter into the embryo sac. One of the gametes fuses with the egg and the other fuses with
the secondary nucleus. The fusion of a male gamete with a female gamete (egg) is known as fertilization. The fertilized egg
is known as zygote which develops into an embryo.
Double fertilization
The other male gamete fuses with the secondary nucleus. The secondary nucleus is diploid in The fusion of this nucleus with the second male gamete is known as triple fusion. The triple fusion nucleus is called primary endosperm nucleus because it
develops into an endosperm. Endosperm is a nutritive tissue meant for the development of the embryo. The process of fusion of a male gamete with an egg and the othergamete with a secondary nucleus is known as double fertilization.Post fertilization changes :
i. The ovule develops into a seed.
ii. The integuments of the ovule develop intoseed coats.
iii. The ovary enlarges and develops into afruit.
FRUIT FORMATION
Fruits form a part of our daily diet. Fruits are rich in vitamins and give us energy. The fruit may be defined as a fertilized and ripened ovary. The ovary wall becomes the fruit wall (pericarp) and the ovule becomes the seed.
Some fruits develop without the act of fertilization. Such fruits are called Parthenocarpic fruits. e.g. seedless grapes,
guava, etc.
Simple fruits
A simple fruit is developed from a single ovary with a monocarpellary or multicarpellary, syncarpous gynoecium.
Based on the nature of the pericarp, the simple fruits are classified into fleshy fruits and dry fruits.
Simple fleshy fruits
In simple fleshy fruits, the pericarp is succulent and juicy when fully ripe. The fleshy fruits are indehiscent in nature. The pericarp is made up of three layers, namely epicarp, mesocarp and endocarp. There are mainly two types of fleshy fruits – Baccate and Drupaceous. Baccate is further classified into berry, hesperidium, pome and pepo.
Simple dry fruits
These fruits have a dry pericarp. They are classified based on mode of dehiscense as dry dehiscent, dry indehiscent and schizocarpic fruits.
Dry dehiscent fruit: These fruits split open at maturity to disperse the seeds.
Dry indehiscent fruit
These fruits do not split open at maturity and the seeds are liberated by the decaying of pericarp.
Schizocarpic fruits
At maturity, these fruits break into many one - seeded parts called mericarps. The mericarps containing the seeds remain indehiscent. Thus the schizocarpic fruits show characters of both dehiscent and indehiscent fruits.
Aggregate Fruit
It is developed from a single flower with a multicarpellary, apocarpous, superior ovary.Each free carpel develops into a fruitlet. Hence, the aggregate fruit has a cluster of fruitlets attached to a common stalk e.g. Polyalthia.
In Annona squamosa (custard apple), the margin of the matured ovaries of carpels after fertilization (fruitlets) are united and appears like a single fruit.
Composite or Multiple fruit
Multiple or composite fruit is formed from all the flowers of whole inflorescence and gives a single fruit. There are two types of multiple fruits namely sorosis and syconus.
SEED FORMATION
The seed is a fertilized ovule. It possesses embryo, food materials and are protectedby the seed coat. During favourable
conditions, the seed germinates and gives rise to a new seedling.
Seeds vary greatly in size, shape, colour and texture. In orchids, there are many seeds which are tiny dust like particles. In coconut, there is a large sized seed. In both cases, the seed grows into a full plant.
Based on the number of cotyledons in the
seed, the angiosperms have been divided
into two groups.
1. Dicotyledons: Seeds with two cotyledons e.g. pea, bean and castor.
2. Monocotyledons: Seeds with one cotyledon e.g. maize, rice, wheat and onion.
1. Structure of a dicot seed (bean)
The seed is bulky, oval and slightly indented on one side. On this side, there is a short longitudinal, whitish ridge called the raphae. At one end of the raphae, there is a minute opening known as germ pore or micropyle.
If a water-soaked seed is pressed gently, a small drop of water along with air bubbles will come out through the micropyle.
The embryo is enclosed by the seed coat. It consists of cotyledons attached to the primary axis which has a rudimentary root
portion called the radicle and a rudimentary stem portion known as plumule.The tip of the radicle projects outside,and is nearer to the micropyle. The plumule is placed between the two cotyledons and consists of a shoot axis and a small bud
having two tiny folded leaves.
2. Structure of monocot seed (paddy)In paddy, the so - called seed is actually a fruit. It is a simple indehiscent one - seeded fruit known as caryopsis (you have already studied about this in the lesson on fruits).The seed coat is very thin. The fruit wall (pericarp) is thin and fused with the seed coat. The fruit is generally covered with yellowish bract and bracteoles which are commonly known as chaff. The embryo consists of a single cotyledon called scutellum and a shoot axis. The lower part of the axis is the radicle, covered by a sheath called coleorhiza (root sheath). The upper part is known as plumule which is covered by a sheath called coleoptile.
In a day or two, after the seed is placed in moist soil, the coleorhiza pierces the base of the seed. The radicle comes out next after
splitting the coleorhiza.
The radicle does not form the root system. Meanwhile, roots are formed from the lower most nodes of the stem. These roots are called adventitious roots. These adventitious roots form the fibrous root system of the matured plant.
DISPERSAL OF SEEDS
The seeds fall far away from the mother plant. Why?The reproductive capacity of plants is so tremendous that a very large number of seeds are produced by a single plant. If all these seeds fall directly below the parent plant, the seedlings would have to compete for space, water, oxygen, minerals and sunlight. When the seedlings are grouped together in one place, they can easily be destroyed by grazing animals. Such a situation would be detrimental to the species.
The fruits and seeds of plants have evolved various devices by which they can be distributed far and wide through various
agencies. This not only eliminates the unhealthy competitive struggle that would arise from overcrowding, but also ensures the successful spreading and establishment of a species on the earth. Most fruits and seeds have evolved adaptations for
dispersal.
Agents for the Dispersal of Fruits and Seeds:
Based on the agents involved in dispersal, there are various types of dispersal mechanisms of fruits and seeds in plants.
Autochory: Autochory is an active mechanism of self dispersal of fruits and seeds. Fruits like balsam burst with a sudden jerk and disperse the seeds by wind through an explosive mechanism.
Anemochoryis the wind dispersal of fruits and seeds.The wind blows them away and for this they have to be light, so that their buoyancy may enable them to float on air over long distances. Some of them are provided with hairs and membranous wing-like structures, which enable them to be carried away easily. e.g. Seeds dispersedby wind are Calotropis (Erukkum), Moringa (drumsticks)etc.
Fruits of Tridax carry a persistent calyx modified into a pappus (a ring of fine, feathery hair) which acts like a parachute and aids in the dispersal by wind.
Hydrochory:Hydrochory is a mechanism in which dispersal of fruits and seeds takes place by means of water. Fruits which are dispersed by water have outer coats that are modified to enable them to float. The mesocarp of coconut is fibrous and is easily carried away by water currents.
The spongy thalamus with air chamber of the Lotus floats in streams of water and after some time, the fruits get separated
and the seeds germinate.
Zoochory: Zoochory is a mechanism in which dispersal of fruits and seeds is by animals. Some fruits are provided with hooks, spines, bristles, stiff hair,etc. on their outer coat. With the aid of these out growths, these fruits stick to the furry coats or skins of some animals and get carried from one place to another. The fruits of Xanthium have sharp-pointed stiff hooks and in the Achyranthus, the perianth and bracts are pointed. Many fleshy fruits are eaten by animals and human beings and the seeds are thrown away.
In fruits like tomato and guava, the seeds are eaten along with the edible portion and are later passed out in the excreta. These types of seeds are protected from the digestive juices by their seed coat.
Man is responsible for the dispersal of many fruits and seeds. In the pursuit of more economy, useful plants like Cinchona, Rubber and Eucalyptus have been successfully introduced by man and they have become acclamatised to the new surroundings far away from their original habitat.