CBSE Notes Class 7 Science Chapter 8 Reproduction in Plants

All living organisms reproduce. What it means in simple terms is that living organisms produce young ones of their own kind. For example, a cat produces kittens which grow into adult cats. In plants too, seeds grow into young seedlings. The seedlings in due course of time develop into mature plants, as in a mango tree. Reproduction is one of the most important properties of living organisms. It means creating new life, producing young ones of their own kind.

1.0Modes of Reproduction 

• Sexual Reproduction - Flowers produce seeds which are formed by the fusion of a male gamete with a female gamete. This type of reproduction is called sexual reproduction.
• Asexual reproduction, on the other hand, is any type of reproduction that does not involve the union of gametes. 
• New individual is produced from a single parent. 
• Methods of asexual reproduction 
• Budding
• Fragmentation  
• Spore formation  
• Vegetative propagation

2.0Asexual Reproduction in Plants 

1. Budding 

• Asexual mode of reproduction in which a small outgrowth (bud) appears on the body of the organism.  Budding is commonly observed in yeast. 
• A bulb-like projection, called the bud, is formed on the body. 
• The nucleus of the body divides into two. 
• Then, one of the two nuclei passes into the bud. 
• The bud detaches itself from the parent body. 
• It grows to full size and becomes a new individual.

2. Fragmentation 

• In some filamentous organisms such as Spirogyra (an algae), the filaments break up into two or more fragments. 
• Each fragment or piece grows into a new individual.

3. Spore formation 

• In non-flowering plants (the plants which do not produce seeds), like fungi (Mucor), bacteria, ferns or mosses, formation of spores is a common method of reproduction. 
• Spores are very small in size. 
• They have thick walls. 
• The thick walls help the spores to survive adverse conditions in the environment, like high temperature, scarcity of water and lack of food. 
• The spores give rise to new organisms under favorable conditions. 

4. Vegetative propagation 

• This is an asexual method of reproduction in plants where vegetative parts namely the root, stem, leaf or buds give rise to new plants. 
• No reproductive organs take part in this method of reproduction and therefore, no seeds are produced. 
• Vegetative propagation by roots — In sweet potato, dahlia or asparagus, the swollen roots are present. New plants arise from these swollen roots buried in the soil.
• Vegetative propagation by stems — A number of plants like potato, ginger, turmeric by stems. 
• The potato which you eat is in fact an underground swollen stem (tuber) which contains stored food material. If you observe a potato with a magnifying glass, you will find "scars". These scars are called 'eyes'. The 'eyes' on the potato tuber sprout (germinate) and give rise to new plants. 

• Vegetative propagation by leaves — In Bryophyllum, plantlets develop from the margins of intact leaves. 
• These plantlets, on being detached, develop into independent plants. 

Advantages of Vegetative Reproduction 

• It is an easier, rapid and less expensive method of propagation. Plants can be grown in much less time. 
• Seedless plants can be raised. 
• Plants produced by this method are identical copies of the parent plant and show no variations.
• Plants like banana, sugarcane, sweet potato, rose and jasmine do not produce viable seeds. Such plants can be easily grown by this method. 

Artificial methods of vegetative propagation 

• Because of the advantages offered by vegetative propagation, humans have used this method for artificial multiplication of useful plants. 
• For this reason, such multiplication methods are called artificial methods of vegetative propagation. 
• Some of these methods are as follows: 
• Cutting : Artificial method of vegetative propagation in which short lengths of plant stem or root are used for growing new plants. 
• Stem cutting : Cuttings are short lengths of the plant which, when removed and placed in soil, with suitable conditions, develop roots and leaves and grow into independent plants. e.g. Rose, money plants, Sugarcane 
• Root cuttings : In certain plants like lemon, tamarind, etc. root cuttings when put in the damp soil give rise to roots and shoots and form new plants.  

3.0Sexual Reproduction in Plants

• A flower commonly has four parts. These parts, in order, from the outside are sepals, petals, stamens and pistils. 
• Sepals : The outermost whorl of the flower consists of sepals. These are the green, leaf-like structures at the base of the flower. Sepals protect the flower during its development and support the flower when it blooms. 
• Petals : The second whorl consists of petals. These are colorful structures that surround the inner parts of flowers. Its function is to attract the pollinators. 
• Stamen : The third whorl of flower consists of stamen. It consists of an anther and a filament. An anther is a swollen structure present on the tip of the filament. The anther produces a powdery substance called the pollen grains. Pollen grains contain the male sex cell or male gamete. Pollen grains are light and can be carried by wind, water or insects . 
• Pistil : It is a single elongated structure in the center of the flower. It consists of a basal swollen portion called the ovary. 
• The ovary continues into a long style and ends in two knob-like parts, the stigma. The ovary contains many ovules. The female sex cell or gamete (egg) is present inside the ovule . 

Bisexual Flower  

• In many plants, the male and the female parts are present in the same flower. 
• Examples —rose, mustard and petunia. 

Unisexual Flower 

• In some, the male and the female parts are born in different flowers. 
• Examples- papaya, corn and cucumber.

Pollination 

• Pollination is the transfer of pollen grains from the ripe anther to the stigma. 
• The transfer of pollen grains to the stigma can take place in two ways : 
• Self pollination : Within the same flower or between flowers of the same plant
• Cross pollination : Between flowers from different plants of the same species.
• Cross pollination often involves various external agencies to carry pollen grains from one flower to another one. These agencies may be air, water, insects or animals. 
• Most flowers are pollinated by insects. 
• Though these insects visit the flowers for nectar or honey, at the same time, they help in pollination. 
• When an insect visits a flower, the pollen grains get deposited on the body of the insect. When this insect visits another flower, the deposited pollen grains now get dusted on the stigma of the second flower, thus bringing about the transfer of pollen grains from the anther to the stigma (pollination). 
• Pollen grains of all flowers are not carried by insects. In some cases, they are carried by wind (wind pollination).  
• In the case of water plants, pollen grains are carried by water (water pollination).

Fertilization 

• Fertilization is a step between pollination and seed formation. 
• The fusion of the male gamete with the female gamete is called fertilization. 
• During fertilization, the following events take place :
• The pollen grains germinate on the stigma and pollen tubes develop. The pollen tubes move downwards into the style. These tubes are the carriers of male gametes.
• One pollen tube finally enters the ovule where the female gamete is located. Female gamete or egg cell is present inside the ovule. 
• Finally the male gamete fuses with the female gamete. This completes the process of fertilization. The fusion product or the cell formed as a result of fusion of the two gametes is called zygote. The zygote soon develops into an embryo (baby plant). 
• The seeds develop from the ovules. The seed contains an embryo enclosed in a protective seed coat. Some fruits are fleshy and juicy such as mango, apple and orange. Some fruits are hard like almonds and walnuts. Functions of fruits 
• The fruit wall gives protection to the seeds and therefore, to the embryo.
• The fruit is a storehouse of food material.
• The fruit helps in the dispersal of seeds.

4.0Dispersal of Seeds and Fruits 

• For a seed to give rise to a new plant, certain favorable conditions are necessary. A seed must fall on a suitable place where favorable conditions are present.
• In some cases like maple, drumstick, the fruit has flat, wing-like light structures. Like madar, these fruits are dispersed by wind. 
• In urena and xanthium, the fruits are thorny and stick to our clothes as we pass by them. These fruits also stick to bodies of animals passing nearby. Thus, animals as well as human beings help in dispersal. 
• In some cases like light seeds of grasses or hairy seeds of aak (Madar) and hairy fruit of sunflowers are carried to far-off places by wind. 
• In the case of coconut which is grown on the sea shore, the fruit is large and fibrous. It falls in water, and being fibrous it floats in water. It is thus carried away from the parent plant by water currents. Thus, seeds and fruits may be dispersed to places away from the parent plant by various means like wind, animals, or water. 
• Some seeds are dispersed when the fruits burst with sudden jerks. The seeds are scattered far from the parent plant. This happens in the case of castor and balsam.

Seed dispersal helps the plants to

• Prevent overcrowding. 
• Avoid competition for water, minerals and sunlight
• Spread to new habitats.