Class 7 Science Reproduction in Plants Chapter Notes

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Reproduction in Plants

A characteristic feature of all living organisms is that they give birth to young ones of their own kind. What would happen if new generations of species were not produced from their parents? The earth would turn out to be barren place with no life. Since the life span of an organisms is limited, they have devised ways and means to multiply their number. This ability of all living organisms to produce new individuals of their own kind is called reproduction. However, reproduction is not essential for an organisms’s survival, but it ensures that the organisms leaves behind more individuals of its own kind, so that the species does not perish from earth.
There are different types of plants growing around us in park, school, garden and in our neighbourhood. Many of these plants bear flowers and fruits. Such plant body consists of two main parts:

i) Vegetative Part: These are concerned with nutrition and growth. They comprise of the roots, stems and leaves.

ii) Reproductive Part: This is essentially concerned with reproduction of the plant. It comprises of the flower. A plant may have both male and female parts borne on the same flower or they may be borne by different flowers.

MODES OF REPRODUCTION
The three most common methods of reproduction among living organisms are
1. Asexual reproduction
2. Vegetative reproduction
3. Sexual reproduction

Asexual Reproduction

This type of reproduction involves the production of a new organism by a single parent. It occurs when there is plenty of food available and conditions are suitable for growth. Asexual reproduction is common in unicellular organisms but is also observed in multicellular plants and animals. There are many forms of asexual reproduction.
a) Fission b) Fragmentation c) Budding
d) Sporulation e) Regeneration

a) Fission: Fission means division of cell into two parts. During fission, two daughter cells of equal size are formed from one parent. This is the simplest and most common method of reproduction seen in unicellular organisms such as Amoeba, Paramoecium, Euglena and bacteria.

i) Binary fission: In binary fission, the fully grown parent cell splits into two halves to produce two daughter cells. Binary fission takes place when food is abundant. In Amoeba (Fig. 1) a unicellular organisms that lives in ponds and puddles, first the nucleus divides into two equal nuclei and then the cytoplasm divides. This results in two daughter amoeba which grow attain full size and split again.

The binary fission may be longitudinal or transverse. Paramoecium (Fig. 2) shows transverse binary fission. The parent cell or individual no longer exists after binary fission in complete.

ii) Multiple fission: During unfavourable conditions like lack of water, excessive heat or high temperature, a thick protective wall develops around the Amoeba cell, called the cyst (Fig. 3). The 
Amoeba splits many times within the cyst to form many small Amoebae. This is called multiple fission. When the cyst finally breaks, several daughter cells are released at once.

b) Budding: Budding is another method of asexual reproduction in which a small bulb like outgrowth appears on the body of the organism called the bud. It grows and may break away from the parent to form a miniature organism. Yeast, corals, sponges and Hydra reproduce by budding (Fig. 4).

 
In yeast, a unicellular organisms, a little cytoplasm accumulates at one end of the cell. The nucleus of the parent cell divides into two and one is sent into the bud. Bud formation occurs very rapidly in yeast cells, to form a chain of yeast cells under favourable conditions. All these chains of buds don’t detach from the parent individual. So a sort of colony is formed.
 
In Hydra, a multicellular organisms a bud appears on the body wall which grows into a full Hydra in a day or two, develops tentacles and mouth and detaches from the parent body.
 
In corals and sponges, buds don’t separate out but remain attached to the parent organism. They grow to full size and reproduce again and again, producing a colony.
 
c) Spore Formation: During unfavorable conditions many fungi (mushrooms), ferns, mosses and bacteria such as yeast, rhizopus and mucor, reproduce through spore formation. A spore is a tiny spherical unicellular body protected by a hard and thick wall (Fig. 5). Spores are microscopic structures. These spores help in overcoming conditions unfavourable for reproduction, in which the plants can survive eg. lack of food and external temperature conditions by forming spore The resistant spores of various organisms, e.g. the bread mould (fungi) float in the air and settle on covered bread or any other suitable food. It germinates and grows under favourble conditions like nutritions, temperature, moisture, etc. to produce hyphae. Hyphae penetrate the bread, as can be seen as white thread like structure on the bread during rainy season some hyphae are erect and possess the swelling at their tips, called sporangium. The spores are produced by multiple fission inside the sporangium.

 
The white powder like substance that grows on leather goods and shoes during the rainy season are also fungi whose spores germinate to produce new colonies on leather given favourable conditions of temperature and moisture.
 
In case of mosses and ferns, spores are produced inside the special structure called capsules.
 
d) Fragmentation: There are several filamentous organisms like Spirogyra or flatworm that increases their population by fragmenting their filament The organisms breaks up into two or more fragments after maturation, and each fragment starts growing into a new individual (Fig. 6).

 
e) Regeneration: The ability of certain plants and animals to re-develop, a lost limb or body part, is called
 
‘regeneration’, e.g. Amoeba and Paramoecium can regenerate lost parts. In fact, if an Amoeba is cut or crushed into many small pieces, with a portion of nucleus in every piece, each piece can develop into a full Amoeba. Regeneration is more common in plants than in animals.

 
If a Hydra (Fig. 7A), earthworm or Planaria is cut into pieces, each piece can develop into small Hydra, earthworm or Planaria. A starfish (Fig.7B ) if caught by the enemy, loses its arms and escapes. Later on, it can develop or regenerate than arm. Similarly, a wall lizard can regenerate and its lost tail. A grasshopper can grow a new leg in place of broken one. The power of regeneration is restricted only up to heating of wounds etc., in man. Humans are not able to regenerate a lost part naturally.
 
Vegetative Reproduction
 
Like spores, higher plants can also survive unfavourable conditions. Under such conditions, the aerial parts of a plant usually die but the underground parts like root or stem remain dormant or inactive. When conditions are favourable again, these dormant parts grow again to produce a new plant. Similar is the case with Colocasia, sweet potato, potato etc., which do not produce any seeds. Man has encashed this property of such plants to maximum advantage. Thus, when new plants are produced from parts of the parent plant such as the root, stem or leaves, without the help of any reproductive organs, it is known as vegetative reproduction. Different methods are used in horticulture for fruit yielding and ornamental plant, like, cutting (stem), eg. in sugar cane, layering and grafting in mango, roses etc.
 
a) Vegetative reproduction by stem: Some plants send out a side branch from the main plant, and a root grows down into the soil. The stem grows in length and creeps along the ground (runners) and forms roots at intervals, thus forming new plants (Fig.8). When the new plant has grown enough, the link between the old and the new plant withers away, eg. some species of grass and strawberry.

 
Some plants reproduce vegetatively beneath the soil, e.g., onion, potato, ginger, Colocasia etc., Modified underground stems also store food. Underground stems are capable of producing several new plants from their buds and thus help the plant to multiply. Tuber, bulb, corn, rhizome are the different modifications of an underground stem. (Fig,8).
 
i) Tuber: Seeds of potato are not viable. Potato tuber is the swelling of the branch of an underground stem. The leaves on the branches and stems growing above the soil make food in the form of starch. This starch is passed down the stem via the phloem tissues, into the underground branches to make new tuber The tubers have ‘eyes’ present on the them. The ‘eye’ is made up of a bud and scale leaves. When sown, each eye is capable of developing the root stem and leaves, as it contains meristematic tissue.
 
ii) Bulb: Bulbs are actually swollen large underground buds covered with scale leave It is a shoot modification that has a very short stem and apical and axillary buds. It stores food for the growth of new leaves and flowers. For example, onion, lily, tulip, garlic etc.
 
iii) Rhizome: It is horizontally growing underground stem, swollen with stored food. A rhizome bears distinct nodes, internodes scaly leaves and adventitious roots. It has buds on its surface from which grow new plants. Ginger and turmeric are examples (Fig.9).

 
iv) Corm: Some plants like Colocasia, Crocus, Gladiolus etc., consist of a short swollen solid fleshy underground stem (Fig. 10). It is in fact a condensed form of rhizome and bears one or more bud The food made by the leaves is stored in the corm at the end of the growing season, until it is used for the growth of a new plant, the following season. Daughter corms develop from the sides of the parent corm, which later break off to form new plants.

 
b) Vegetative Propagation by Root: Some plants like Dahlia and sweet potato have roots that help in vegetative propagation (Fig.11). If roots tubes of seet potato is cut into pieces and sown in well prepared soil, a new plant can be produce.

 
c) Vegetative Propagation by Leaves: In Bryophyllum and Begonia a new plants are produced from leaves, which have buds on the notches in their margins (Fig.12). These buds after falling on the ground or coming in contact with the soil, grow into new plant. Thus a new plant with proper root and shoot system can grow from a bud.

 
d) Vegetative propagation using artificial methods: Vegetative propagation is a simple, fast and less expensive method of plant propagation It is, therefore, commonly, used in horticulture and agriculture. The techniques used are stem cutting, layering, grafting and tissues culture.
 
i) Cutting: In this method, cutting of a healthy young branch of a plant having leaf buds is planted in the moist soil. Cutting develops roots and grows into a new plant. This method is to propagates plants like chameli, Bougainvillea, rose and sugarcane (Fig.13).

 
ii) Layering: In this process a young branch is bent towards the ground and covered with moist soil forming a layer. After some time, roots develop from the covered part. The branch is then cut off from the parent plant and allowed to grow into a new plant. This method is commonly used by gardeners to develop plants like jasmine, vines rose and Boughainvillea (Fig.14).

 
iii) Grafting: This is a common method used in horticulture to develop new varieties of ornamental plants and fruit trees.
 
• In this method a bud or cutting with buds of one plant, called the scion, is kept over the cut stem of another plant, called the stock.
 
• The scion and the stock are then firmly tied together.

 
• The care is to be taken that stock has an extensive root system under the soil.
 
• After sometimes, the tissue, of the stock and scion join together to form one plant. The stock supplies the essential nutrients to the scion (Fig. 15)
 
This is the technique of combining the features of two plants. For example a high yielding variety may be grafted to a disease resistant variety to have characteristics of both the plants. Many new varieties of mangoes available in the market are developed by this method.
 
iv) Tissue Culture: It is a modern method of veget ative propagation of plants like Chrysanthemum, orchids, Asparagus, etc. In this method cells from the growing tip of a plant are taken. They are placed in a nutrient medium contains hormones that make the cells divide and form group of cell These small groups of cells grow roots. The plantlet is then shifted to another medium that contains hormones suitable for growing roots. Finally the small plants are then grown in pots of soil (Fig. 16). When the plants are big enough, they are transplanted in the fields. By this technique a very large number of plants can be produced from just one parent plant. Tissue culture is being used very successfully in getting high yielding dwarf and disease resistant varieties of paddy.

 
Advantages of Vegetative Reproduction
 
1. It is a sure and quick method of multiplying a plant.
2. Plants grown this way require less time to mature and bear fruits than those grown from the seed
3. It helps in rapid spread of the plant over a area.
4. The new plants produced by this method are exactly like the parent plant.
5. Through grafting, plants with desirable qualities of two varieties can be combined.
6. Plants developed by vegetative propagation usually need less attention than plants grown from seed
7. The survival rate of plants is almost 100% in vegetative reproduction while it is hardly 1% through the formation of seed
 
Disadvantages of Vegetative Propagation / Reproduction
 
1. It leads to overcrowding.
2. There is no mechanism or dispersal
3. The plants may show degeneration due to the absence of sexual stimul
4. Plants so propagated are not so efficiently protected as the seeds are. They decay easily.
5. Due to lack of genetic variation, the adaptability of plants to the environment is limited. Plants gradually lose vigour and become prone to disease
 
Sexual Reproduction
 
Most flowering plants are hermaphrodites i.e., both male and female gametes or sex cells are present in the same flower. Thus flower are the reproductive organism plants. Flowers of plants like china rose, Calotropis and banyan contain both male and female gamete. Plants use various methods to ensure fertilization and sexual reproduction. For example, moss grows in damp places because it needs water for the sperms to swim to the eggs to fertilize them.
 
a) Flower: It is the reproductive organ of a plant. A flower is attached to the plant by stalk. There are many kinds of flowers but the main function of the flower is sexual reproduction and to produce seed which, on getting favourable conditions, produce new plant Certain common features are present in all flowers. A flower that contain both male and female parts is known as complete flower. Thus, a complete flower contains sepals, petals, androecium or stamen and gynoecium or carpel or pistils. The androecium or stamen produces the male gametes while the gynoecium or pistil produces the female gamete (Fig. 17).

 
Some flowers like corm and papaya contain either male or female reproductive organs and can produce only one type of gamete are called incomplete flowers.
 
There are four main parts of the flower.
 
1. Sepals 2. Petals 3. Stamen 4. Pistil
 
Sepals: The sepals are green in colour and protect the flower in bud condition.
 
Petals: The petals are large and variously coloured to attract insect for pollination.
 
Stamen: It is the male reproductive part and consists of two parts the long narrow salts like filaments and the upper, broader knob like anther. The anther lobes consist of pollen sacs that contain, millions of pollen grains, which are yellow in colour. The male gametes are produced inside the pollen grains.
 
Pistil: It is the female reproductive part and consist of a swollen stigma at the top, a slender tube like style and a swollen ovary at the bottom. Inside the ovary, the ovules contain a little bag called the embryo sac. The unripe seeds or eggs are present inside this sac.
 
b) Pollination: When the anther is mature, it splits open and sheds the pollen grain They are dispersed by various agencies such as wind and water, so that they can be transferred to the stigma of another flower of the same species or of the same flower. The pollen grains have to be deposited on to the stigma fertilize the egg in the ovary. Thus, the transfer of pollen from anther to stigma is called pollination. Pollination takes place through wind, water and insects.
 
c) Types of Pollination: There are two types of pollination
 
1. Self pollination: When the pollen grains are transferred within the same flower or between the flowers of the same plant. It is further of two type
 
i) Autogamy: Transference of pollen grains from anther of a flower to the stigma of the same flower, eg. pea, wheat, rice etc.
 
ii) Geitonogamy: Transference of pollen grains from anther of a flower to the stigma of another flower borne on the same plant.
 
2. Cross Pollination: When the pollen grains are transferred between the, flowers of difference plants of the same species or a very closely related species (Fig.). It is also called allogamy. Cross-pollination needs a medium or an external agency to perform pollination (Fig. 18).

 
Depending on the medium, cross pollination is further of 5 types.
 
i) Wind pollination or anemophily: Flower of wheat, rice, maize and rye grass are pollinated by win Wind pollinated flowers have the following characteristics.
 
• They are not brightly coloured and are usually white.
 
• They do not produce nectar.
 
• They produce small and light pollen in large number to be carried away by wind.

 
ii) Water pollination or hydrophily: Pollination in aquatic plants is usually carried out by water.
 
The examples of water pollinated plants are seagrass, Hydrilla and Vallisneria. Water pollinated flowers release their pollen grains into the water. These pollens are passively carried to other flowers by water currents for pollination (Fig. 19).
 
iii) Pollination by insects or entomophily: Insects like honeybees, wasps, moths and butterflies visit flowers for nectar. While collecting nectar, some pollens stick to the body of these insects. When these insects visit another flower the pollen grains are brushed off, some of which fall on the stigma.
 
Insect pollinated flowers have following characteristics::
 
• They usually have nectaries that produce nectar.
 
• They have sweet smell and bright colours
 
• They have sticky stigma and sticky pollen grains
 
Examples of insect pollinated flowers are sweet pea, orchids, buttercup, sunflower etc. iv) Bird pollination or ornithophily: Sunbir ds and hummingbirds are two large groups of birds which pollinate flowers. Hummingbirds however in front of flowers while sipping the nectar. Sun birds sit on the flower stalk to collect the nectar (Fig. 20). Bird pollinated flowers have following characteristics.
 
• They are not scented because birds do not h a v e a well developed sense of smell.
 
• They are of bright red, yellow or orange in colour to attract the birds.

 
v) Bat Pollination or Chiropterophily: Some mammals like bats and rodents also pollinate flowers Mammal, pollinated flowers have the following characteristics.
 
• Flowers are quite sturdy in order to bear the vigorous activity of small mammals, while feeding on the nectar of these flowers.
 
• They are usually brown or white in colour.
 
• They often have a strong scent
 
Bats being nocturnal animals can transport pollens over long distances (upto 30 km)
 
Advantages of self pollination
 
1. Flowers need not be large or conpicuous
2. Scent and nectar are generally not produced by flowers to attract insect.
3. Small quantity of pollen is sufficient
4. Parental characters are preserved from generation to generation.
5. It is a definite method of seed formation.
 
Disadvantage of self pollination
 
1. Week characteristics of the species are passed from one generation to the other.
2. The plant doesn’t improve genetically.
3. Continued self-pollination may decrease vigour of the future generations of the specie
 
Advantages of Cross Pollination
 
1. New and better adapted varities are produced continuously.
2. More vigorous offsprings are produced.
 
Disadvantages of Cross Pollination
 
1. The process takes a lot out of the plant as the flowers have to be large, showy with bright petals and nectar to attract insects for ensuring pollination. This costs the plant a heavy amount of food and energy to produce.
2. Large quantity of pollens are produced and wastage is substantial.
3. Pollination is dependents on pollinating agencies or medium like water, air, insects, bats, birds animals, etc.
4. Pollination is an uncertainty; it is not definite.
 
Table 1: Differences between self pollination and cross pollination
 

Table 2: Differences between wind pollinated and insect pollinated flower.
 

 
FERTILIZATION
 
The process of fusion of a male gamete - pollen - with the female gamete inside the ovary of the flower to form a single - celled zygote is called fertilization (Fig. 21).
 
Events after Pollination
 
In pollen gains nectar is a liquid containing sugar. It is secreted by the stigma of each flower. Pollen grains respond to this liquid and start growing. But, the pollen grains start growing only if the liquid is produced by a flower of its own species. If the pollen land on the stigma of a flower of a different species, fertilization will not occur and the pollen will go waste.

 
A mature pollengrain is a double walled structure. The hard and rough outer wall is called exine while the soft and membranous inner wall is called, intine. After pollination, the intine of the pollen grain from pollen tube through weak areas or pores in the pollen wall (the germ pores). Nectar stimulates the growth of pollen tube. The pollen’s nucleus is divided into two parts - a tube nucleus and nucleus in the germinative cell. The tube nucleus remains in the cytoplasm, which the other one forms an eccentric crescent. The pollen tube with two male gametes and tube nucleus runs through the style by dissolving the stylar cells with enzymes. It turns towards the micropylar end of the ovule in the ovary cavity. On reaching the ovule, the male gametes or nuclei or sperm nuclei are released. The length of the pollen tube depends on the length of style (Fig,.22).

 
In the Ovule
 
The ovule within the ovary also undergoes many developmental changes before receiving the male gametes. Two layers called integuments surround the ovule and form an opening at one end, the micropyle (Fig. 23)
 
The inner side of the ovule consists of the nucellus, the nurishing tissue, at the centre of which lies the embryo sac. On piercing the nucellus, the pollen tube penetrates the embryo sac and reaches the egg apparatus. The tube passes either between the egg and synergids or between one synergid and wall of the embryo sac. Ultimately, the tip of the pollen tube bursts and two male gametes are discharged. The tube nucleus disorganizes before bursting the pollen tube.

 
Inside the embryo sac, one cell divides three times to produce eight nuclei, whereafter growth stops abruptly. Out of t hese one b ecomes t he egg nu cleu s a nd female gametophyte, which is located near the micropyle.
 
One of the male gametes fuses with the egg cell or oospore (syngamy) causing fertilization, as a result of which a diploid oospore or zygote is formed. The other gamete fuses with the secondary nucleus forming a triploid endosperm nucleus which later on give rise to endosperm. This fusion of two polar nuclei with one male nucleus is known as triple fusion.
 
Further growth of embryo sac occurs only after the zygote formation and primary nucleus has been created by triple fusion. This initiates the formation of endosperm only when needed. Endosperm provides nutrition for the developing embryo.
 
Double fertilization
 
Double fertilization refers to when flowering plants contain two sperm cells that each fertilize a different cell. The cells both reside in the overy. After the pollen tube reaches the ovary the pollen tube nucleus disintegrates and the two sperm cells are released into the ovary; one of the two sperm cells fertilises the egg cell )at the bottom of the ovulenear the micropyle), forming a diploid zygote. Which develops into the embryo, or seed, this is the point when fertilisation actually occurs. Note that pollination and fertilisation are two separate processes. At the same time, the other sperm cell fertilizes the central cell nuclei, creating a triploid cell, which develops into endosperm. The purpose of the endosperm is to provide nutrients to the developing embryo
 
Benefits of double fertilization
 
The benefit of double fertilization is, it produces on one side ovule and on the other hand it provides nutritional endosperm.
 
One possible advantage of double fertilization is that the plant does not invest energy in seed nutritive tissue until after an egg has been fertilized. Another possible advantage is that the endosperm nucleus is very active and divides rapidly. It forms the nutritive tissue very quickly. Rapid seed development has obvious advantages.
 
The Floral Parts
 
The flower, being the reproductive organ of a plant, loses its utility after fertilization and formation of zygote has occurred. The zygote develops into the embryo. The embryo comprises a radicle (seed root), plumule (seed shoot) and one or two cotyledons, which store food for germination. The flower usually withers away along with the petals, while the stamens, style and stigma fall off. The calyx may either fall off or stay on the stalk. The ovary swells and grows to form the fruit and the ovules develop into seeds.
 
The seed hardens and dries. It enables it to survive in adverse conditions. The ovary wall may also harden and become a pod (eg. poppy) or it may become fleshy and succulent (eg. tomatoes, plums).
 
At maturity the seeds contained within the fruit are released, dispersed and on getting favourable conditions
 
- germinate to form new plants. These represent the next generation of the mother plant.
 
Fruit
 
The fruit is formed by stimulus of fertilization. A fruit is a fully developed ovary with its contents. They may or may not contain seeds. At times, the fruit develops even without fertilization. This is called parthenocarpy. thus, parthenogenesis is the process of formation of an embryo directly from the cell or a male gamete without fusion of egg and sperm.
 
Dispersal of Seeds and Fruits
 
Plants can’t move around like animals. So they have no means to spread their fruits and seeds. In such a situation, these will fall only on the ground beneath the plant. The young plants will thus compete with each other and expansion of the species with also be severely limited.
 
Fruits and seeds have devised or adapted certain ways to overcome this condition and to enhance the expansion of species. Fruits and seeds are carried away by wind, water insects, birds animals or by their own dispersal mechanisms. The scattering of seeds over a large area by the agents of dispersal is called dispersal of seeds.
 
i) Wind: The seeds have to be small light and have structures like wings, hair like filaments etc. to be blown away by wind. Some fruits and seeds show parachute mechanism, flattened fruits and censer mechanism, etc. All these adaptations help the seeds to be carried by the wind.
 
a) Wings: For example drumstick and maple seeds have wings to facilitate their dispersal.
 
b) Hair: For example, seeds of madar and cotton have hairs or tufts of hairs to catch the breeze and be blown away.
 
c) Parachute mechanism: For example in Dandelion and Tridax the calyx is modified and has hairy growth for seed dispersal. The persistent calyx forms a balloon - like structure in case of physali
 
d) Flattened fruits: For example, sheesham and albizzia have flattened fruits which are light so that they can be blown away by wind.
 
e) Censer mechanism: For example, tori and datura fruits are perforated to release seeds by swinging in air.
 
ii) Animals: Seeds bear hooks or spines which help them to snag the fur of animals or our clothes, and get carried for away . Some seeds eaten along with the fruits, pass out unharmed in the waste, to germinate where they fall.
 
a) Edible fruits: For example, seeds of fruits like apple, mango, guava, etc., are dispersed by man and animal Birds also play a vital role in seed dispersal of edible fruits.
 
b) Stored seeds: Some animals carry seed to their place of shelter where seeds may get suitable conditions to germinate into new plant
 
c) Hooked seeds: For example, Xanthium tiger nail stick to the fur of the animals and get carried away from the mother plant.
 
d) Sticky seeds: For example, water melon and cucumber have a sticky outer layer and get stuck to the animals body and are carried to other place

iii) Water: For example coconut, lotus, etc., are plants that grow on sea shores and lakes. These can float on water to travel long distances before getting, deposited at favourable places for germination. The thick fibrous coat of a coconut and its hollow structure enables it to float in water. Similarly, lotus fruit has a spongy part that helps in flotation.

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