(a.) Juvenile phase.
(b.) reproductive phase.
(C.) senescent phase.
4.Events in sexual reproduction.
(a.) pre- fertilization event.
(c.) post fertilization event.
The period from birth to the natural death of an organism represents its life span.
Life span are not correlated with size or complexity.
Life span of some organism:–
It is the biological process by which new individual organism “offspring” are produced from their parents.
Each organism exist as a result of reproduction:-
Such a group of morphologically and genetically similar individual is called clone.
4. It is simple and quick method of reproduction.
Asexual reproduction also occur through formation of specialized reproductive structure like spores. The most common spores are zoospores and conidia.
Zoospores:- These are microscopic motile spore that move by means of flagella. It is formed inside sporangia Example :- chlamydomonas
Conidia :-These are non motile spore produced by special branches called conidiophores. It occur outside over conidiophores Example :- penicillium
It is a method of multiplication in which a somatic part of the plant detaches from the body of the mother and develops into a new independent plant under suitable environment condition.
The detachable somatic part involved in vegetative propagation is called vegetative propagules.
Sexual reproduction involves formation and fusion of gametes to form the zygote which develops to form the new organism.
It is a reproduction in which new individuals are formed from two parents.
There are 3 phases:
1.Juvenile phase –
2.Reproductive phase –
3.Senescent phase –
It is the phase or period when an organism sexually reproduce.
Following changes takes place in mammals.
Menstrual cycle: cyclic changes in females of primates during phase.
Oestrous cycle: cyclic changes in females of non- primates during reproductive phase.
These are the events in sexual reproduction which occur prior to the fertilization.
It involves two main events:
1.GAMETOGENESIS: It refers to the formation of gametes male and female.
Categories of gamete:-
i) Isogametes- When the fusing gametes are morphologically similar, They are known as isogametes or homogametes. Example :– some of algae and fungi Similar
ii) heterogametes- When the fusing gametes are morphologically distinct type they are known as heterogamete or also known as anisogametes. In case of heterogametes, the female gamete is called egg or ovum and the male gamete antherozoid or sperm.
Example:- fucus, rhizopus etc
Does sexual reproduction always mean gametes from two different individuals?
No, the gametes do not come from two different individuals all the time. In plants, both male and female reproductive structures might be present in the same flower or both male and female flowers might be located on the same plant. In both the cases, the condition is called bisexual or monoecious. Monoecy promotes self fertilization. The gametes for sexual reproduction are coming from the same individual.
For example, coconuts and cucurbits exhibit monoecy.
Similarly, if the male and female flowers are located on different plants, them such a condition is called unisexual or Dioecy prevents self fertilization and promotes cross fertilization.
For example:- Date palm and papaya exhibits dioecy. In the case of animals, the bisexual condition is often referred to as hermaphrodites. Earthworms and leeches are examples for hermaphrodites.
Mitosis or Meiosis:-Gametes are always haploid. The parental body that produces these gametes might be haploid or diploid in nature.
IN HAPLOID ORGANISM:- Gametogenesis is all about mitosis These organisms produce gametes by mitosis. Haploid parental body is more common in the case of organisms with relatively simple organizations. In general, majority of the monerans, fungi, algae and bryophytes posses haploid parental body and hence, produces gametes by mitosis.
IN DIPLOID ORGANISM:-In the case of diploid organisms, reduction division or meiosis is a must, as the gametes are always haploid in nature. Thus, diploid organisms produce their gametes by meiosis. The remaining categories of the plant kingdom, that is, pteridophytes, gymnosperms and angiosperms along with majority of the animals posses diploid parental body. Thus, they require meiosis for the production of gametes. A unique characteristic property of diploid organisms is the presence of meiocytes. ca Meiocytes are specialized cells found in diploid organisms, that undergo meiosis to form gametes.
GAMETE TRANSFER:- After the formation of male and female gametes, compatible gametes must be brought together to facilitate fusion (fertilization or syngamy).
In some cases like fungi and algae, both the male and female gametes are motile and travels towards each other for fusion.
However, in majority of the cases, the female gametes are non-motile and male gametes, that are motile, travels towards gametes for fusion.so there is an need of medium through which, male gametes move.
In the case of flowering plants, both male and female gametes are non-motile) In this case, the non-motile male gametes are transferred to the non-motile female gametes with the help of pollen grains and pollen tube. Each and every transfer of the gametes is done to facilitate the ultimate event of sexual reproduction, that is, fertilization.
pollen grains are the carrier of male and female which are produced in anther are transferred to stigma of carpel through the process of pollination.
i)Self pollination – Transfer of pollen grains from anther to stigma of same flower or different flower of same plant
ii) Cross pollination- Transfer of pollen grains from anther to stigma of different flower of other plants
Fertilization refers to the process of fusion of male and female gametes to form zygote. These process is also known as syngamy.
On the basis of syngamy, fertilization is of two types-
i ) External fertilization- In most aquatic organisms, syngamy occurs in water, that is, in an external medium. This type of fertilization is called external fertilization. Organisms exhibiting external fertilization produces and releases large number of gametes into the external medium. This is done to increase the chances for syngamy to occur. Most of the fishes and amphibians along with majority of the algae, exhibits external fertilization. However, this type of fertilization always poses a great threat to the off-springs, as they are highly exposed to predators.
ii .) Internal fertilization- Fertilization is found to occur inside the body of the organism, rather than in an external medium. Therefore, this type of fertilization is called internal fertilization. It is a common feature of every organism exhibiting internal fertilization, that the female gamete is formed inside the body of the female parent. The male gametes are physically brought near the female gametes for fertilization.
Post-fertilization events refers to all the events of sexual reproduction taking place after fertilization, that is, after the formation of zygote. The major event taking place in this stage is embryo-genesis, that is, development of the embryo.
i) zygote:- For organisms exhibiting external fertilization, zygote is formed in the external medium. However, for organisms exhibiting internal fertilization, zygote is formed inside the body of the organism.
Zygote is the vital link between two successive generations which ensures the continuity from generation to generation.
ii) embryogenesis:- it is the process of development of embryo from zygote. First one is cell division, that adds more cells to the zygote. The second process is cell differentiation, that helps the new cells formed by cell division to undergo specific changes and thereby converts them into specialized tissues and organs.
In flowering plant, zygote develops into embryo. The food for development of embryo comes from o special tissue known as endosperm ultimately the fertilized ovule mature into seed. After fertilization ovary develops into fruits and fruit develops a special fruit wall called pericarp and other parts like sepals, petals and androceium wither or fall off.