How do Organisms Reproduce?

CONCEPTS

1. In asexual reproduction, certain body cells undergo repeated mitotic divisions and give rise to two or more new organisms of the same kind. 

2. Different methods of asexual reproduction are — fission, budding, fragmentation, spore formation, regeneration, vegetative propagation, parthenogenesis and tissue culture. 

3. Fission is a process of division of a single celled organism into two or many cells (called binary and multiple fission respectively), e.g., Amoeba. 

4. Budding is a method of reproduction in which a protuberance develops on the mature organism’s body, attains full maturity and then detaches, e.g., Yeast and Hydra. 

5. Spore formation : Spores are special cells formed within special structures called ‘sporangia’ that disseminate and can form the entire plant. This is exclusive to the plant kingdom, e.g., mushroom, ferns and mosses. 

6. Vegetative propagation is a method of reproduction in which a vegetative plant part (i.e., a nonreproductive part or a non-flowering part) produces a new plant. 

7. For artificial propagation by man, the commonly used methods are : 

(i) Cutting in which a stem is given an oblique cut and the cut stem is kept under appropriate conditions to give rise to an entirely new plant, e.g., Rose and China rose. 

(ii) Layering is a process in which the stem, while still attached to the mother plant is burried under the soil for a while, till it strikes new roots. It is then detached from the mother plant, e.g., Jasmine. This is called ‘soil layering’. The other form of layering is ‘air layering’ in which incision is made on the bark of a tree and covered with moist soil. When rooting takes place in the cut portion, the entire portion is cut and used for propagation. 

(iii) Grafting is a process in which two parts of different plants are joined by bandaging them tightly. This allows cambial activity to unite the two portions. The one that contributes the root is called ‘stock’ and the one that contributes the shoot is known as ‘scion’. Bicoloured roses and different varieties of mangoes are made by this method. 

8. Advantages of Vegetative Propagation 

(i) It allows quicker and easy propagation. 

(ii) Better qualities of the plants can be maintained and the quality can even be enhanced as in seedless oranges. 

(iii) It results in propagation of those plants which do not produce viable seeds or produce seeds with prolonged period of dormancy. 

9. Disadvantages of Vegetative Propagation Most of these propagated plants do not produce viable seeds and hence curtail natural reproduction. 

10. Regeneration is the ability of an organism to replace or repair any lost part. Sometimes, an entire organism can be made from its fragmented body, e.g., Planaria. When an unfertilised egg can be made to develop into an entire organism, it is known as ‘Parthenogenesis’ in animals and ‘Parthenocarpy’ in plants. 

11. The reproductive part of a plant is its flower. A complete flower has four whorls —sepals, petals, androecium and gynoecium. 

12. Male gametes are the pollen grains produced by the anther lobes and the female gametes are the ova within ovules present inside ovaries. 

13. The transfer of pollen grains from the anther to the stigma is known as pollination.

14. Pollination

15. Upon pollination, pollen gets deposited on the stigma. The pollen germinates by forming pollen tubes. Each tube carries two male nuclei at its tip. 

16. The ovary bears ovules. The number of ovules vary from one (as in mango) to many (as in orchid). 

17. During syngamy, the first male nucleus fuses with the ovum to form the zygote. The second male nucleus fuses with the polar nucleus to form the endosperm. 

18. The entire process is summarised as “double fertilisation” and triple fusion. 

19. The fertilised ovule develops into a seed and the fertilised ovary into the fruit. 

20. Sexual reproduction necessarily requires two gametes usually different from one another. Male gamete is the spermatozoa and female gamete is the ovum. 

21. In most animals single individual produces only one type of gamete and hence is unisexual. However, there are organisms that possess both testicles and ovaries and are called bisexuals or hermaphrodites, e.g., earthworm and leech. 

22. The fusion of the two gametes is known as ‘fertilisation’. 

23. Sexual reproduction provides wider scope for variation between the parents and the offsprings. 

24. Male reproductive system in human consists of the following organs — 

(i) Testes in the scrotum 

(ii) Epididymis 

(iii) Vas deferens 

(iv) Urethra 

(v) Penis 

(vi) Accessory glands (seminal vesicle and prostrate gland)

25. Female reproductive system in humans consists of the following organs — 

(i) Ovaries 

(ii) Fallopian tubes or oviducts 

(iii) Uterus or womb 

(iv) Vagina 

26. The attainment of sexual maturity is known as puberty. 

27. The gonads control and regulate three major functions : 

(a) Produce the gametes as well as the sex hormones (testosterone is produced by the testicles and estrogen and progesterone by the ovaries). 

(b) They help in the functioning of the accessory sex organs (i.e., organs which are part of the reproductive system but do not directly produce gametes). 

(c) They help in the development of secondary sex characters like breast development in girls and growth of facial hair in boys. 

28. In male the testicles are lodged in the scrotum outside the body as the process of sperm formation requires a temperature lower than the normal body temperature. The other parts of this system are : 

(a) Vas deferens 

(b) Urethra, a common passage for the release of urine as well as sperms. 

Testicles function throughout the life of a man, from puberty onwards, though the quality of the sperms declines with age. 

29. The female puberty is considered to be attained with the first menstrual discharge and this is called ‘Menarche’. The stoppage of menstruation marks the end of the reproductive life in females and is known as ‘Menopause’. 

30. The ovaries at birth contain groups of cells called follicles, which mature one at a time throughout the reproductive life of a female in a cyclical manner. Out of a group of follicles only one matures as an ovum or egg. The other cells in the group serve as nourishers. Upon attaining maturity the egg is expelled into the oviduct also known as the fallopian tube. 

31. Population control involves measures by which fertilisation is prevented. The three common methods are : 

(a) Physical barrier methods like condoms or placing of IUCDs (Intra Uterine Contraceptive Devices) in the uterus of the female. 

(b) Chemical methods like use of oral or vaginal pills. 

(c) Surgical methods which involves a reversible surgery in which the tubes conducting the gametes are cut and tied. It is known as vasectomy in males and tubectomy in females. 

32. STDs are sexually transmitted diseases, also called Veneral Diseases (VDs). They are transmitted from the infected individuals to healthy ones during sexual contacts. Gonorrhoea and syphilis are common STDs. AIDS (Acquired Immune Deficiency Syndrome) is also transmitted by sexual contact. Responsible sexual behaviour and prudence can help to prevent the spread of STDs. Both government (National Population Policy and National Health Policy) and non-government organisations are endeavouring to control the fertility rate.