SEXUAL REPRODUCTION

The reproduction in which the gametes of two parents fuse to give rise to a zygote is called sexual reproduction.
Male and female gametes are formed either by the same individual or different individual of the opposite sex.
Because of the fusion of male and female gametes, sexual reproduction results in off springs that are not identical to the parents.
All the sexually reproductive organism have three phases in their life-
Juvenile phase- the period of growth from birth to reproductive maturity in animalsis called juvenile phase and in plants this inter flowering period is known as vegetative phase.
Reproductive phase- the phase in which the organisms reproduce sexually is called reproductive phase.
Senescent phase- the phase between reproductive maturity and death of an organism is called senescent phase, characterized by gradual deterioration in the body leading to the death of an organism.

Fig. phases of growth in a hibiscus plant

Based on the life span of plants, plants can be differentiated into 3 types-

Annual plants- live for one year and start flowering at the end of the year.
Biennuals plants- live for two years and start flowering in the second year.
Perennial plants- live for several years but flower only once in their life time.

Example- bamboo species flower only once in 50-100 years.

Strobilanthuskunthiana flowers once in 12 years.

Fig. strobilus plant

Many mammals reproduce only during favorable season in their reproductive phase and hence these organisms are called as seasonal breeders. Example- cow, buffalo.
Some mammals actively reproduce throughout their reproductive phase and hence are called as continuous breeders. Example- man, rabbit.
Some cyclic changes occur in the female reproductive system of female mammals which is called as reproductive cycle.
Reproductive cycle is of two types –

Menstrual cycle- the cycle of primates in which changes carry on all the year with one ovulation in a month is called menstrual cycle. Example- apes, humans.
Oestrous cycle- the reproductive cycle in non-primate females is called oestrous cycle which consists of a short period during which female receives the male for mating followed by the events of sexual reproduction Example- cow, tiger etc.

Events in sexual reproduction

Events of sexual reproduction is grouped under three categories-
Pre-fertilization events-
The events of sexual reproduction which occurs before fertilization are called pre-fertilization events.
Pre- fertilization events are a) gametogenesis and b) gamete transfer.
Fertilization events- The process of fusion of male and female gametes to form a zygote is called fertilization or syngamy.

Fig. fusion of sperm and egg

Post- fertilization events-

A diploid zygote is formed as a result of fertilization and later the zygote develops into a new organism.
The process of development of embryo from the zygote id called embryogenesis.