`color{blue}"Sexual Reproduction"`: This is a mode of reproduction in which `color{violet}"male and female gametes"` are formed (carrying half the number of chromosomes) which `color{violet}"fuse"` to form the `color{violet}"zygote"` which develops to form the new organism.

`color{green} ✍️ color{green} mathbf("KEY CONCEPT")`

● Sexual reproduction involves formation of the `color{Violet}"male and female gametes"`, either by the same individual or by different individuals of the opposite sex.

● These gametes `color{Violet}"fuse"` to form the `color{Violet}"zygote"` which develops to form the `color{Violet}"new organism"`.

● It is an `color{Violet}"elaborate, complex and slow"` process as compared to asexual reproduction.

● Because of the `color{Violet}"fusion"` of male and female gametes, sexual reproduction results in offspring that are `color{Violet}"not identical"` to the parents or amongst themselves.

● A study of `color{Violet}"diverse organisms"`–plants, animals or fungi–show that though they differ so greatly in `color{Violet}"external morphology"`, `color{Violet}"internal structure"` and `color{Violet}"physiology"`, when it comes to sexual mode of reproduction, surprisingly, they share a `color{Violet}"similar pattern"`.

`color{blue}"Juvenile Phase"`: It is the period of growth in an individual organism `color{violet}"after its birth"` and before it reaches `color{violet}"reproductive maturity"`.

`color{green} ✍️ color{green} mathbf("KEY CONCEPT")`

● All organisms have to reach a certain stage of `color{Violet}"growth and maturity"` in their life, before they can `color{Violet}"reproduce sexually"`.

● That period of growth is called the `color{Violet}"juvenile phase"`. It is known as `color{Violet}"vegetative phase"` in plants.

● This phase is of `color{Violet}"variable durations"` in different organisms.

`color{blue}"Reproductive phase"`: It is the period when an individual organism `color{violet}"reproduces sexually"`.

`color{green} ✍️ color{green} mathbf("KEY CONCEPT")`

● The reproductive phase is also of `color{Violet}"variable duration"` in different organisms.

● The `color{Violet}"end"` of juvenile/vegetative phase which marks the `color{Violet}"beginning"` of the reproductive phase can be seen easily in the `color{Violet}"higher plants"` when they come to `color{Violet}"flower"`.

● In animals, the juvenile phase is followed by `color{Violet}"morphological and physiological changes"` prior to active reproductive behaviour.

● The females of `color{Violet}"placental mammals"` exhibit `color{Violet}"cyclical changes"` in the activities of `color{Violet}"ovaries"` and `color{Violet}"accessory ducts"` as well as `color{Violet}"hormones"` during the reproductive phase.

● In `color{Violet}"non-primate mammals"` like cows, sheep, rats, deers, dogs, tiger, etc., such cyclical changes during reproduction are called `color{Violet}"Oestrus cycle"` where as in `color{Violet}"primates"` (monkeys, apes, and humans) it is called `color{Violet}"Menstrual cycle"`.



● Many other mammals are `color{Violet}"reproductively active"` `color{Violet}"throughout"` their reproductive phase and hence are called `color{Violet}"Continuous breeders"`.



● Many mammals, especially those living in `color{Violet}"natural, wild conditions"` exhibit such cycles only during `color{Violet}"favourable seasons"` in their reproductive phase and are therefore called `color{Violet}"Seasonal breeders"`.

`color{blue}"Senescent phase"`: It is the period when an organism `color{violet}"grows old"` and loses the `color{violet}"ability to reproduce"`.

`color{green} ✍️ color{green} mathbf("KEY CONCEPT")`

● The `color{Violet}"end of reproductive phase"` can be considered as one of the parameters of `color{Violet}"senescence or old age"`.

● There are `color{Violet}"concomitant changes"` in the body (like slowing of metabolism, etc.) during this `color{Violet}"last phase"` of life span.

● `color{Violet}"Old age"` ultimately leads to `color{Violet}"death"`.

● In both plants and animals, `color{Violet}"hormones"` are responsible for the `color{Violet}"transitions"` between the three phases.

● `color{Violet}"Interaction"` between `color{Violet}"hormones"` and certain `color{Violet}"environmental"` factors regulate the `color{Violet}"reproductive processes"` and the associated `color{Violet}"behavioral"` expressions of organisms.

`color{Brown}"PLANTS"` :

● It is the reason why the availability of fruits like mango, apple, jackfruit, is `color{Violet}"seasonal"`.

● Plants–the `color{Violet}"annual and biennial"` types, show clear cut `color{Violet}"vegetative, reproductive and senescent"` phases, but in the `color{Violet}"perennial"` species it is very `color{Violet}"difficult"` to clearly define these phases.

● A few plants exhibit unusual flowering phenomenon:
`star` Some of them such as `color{Violet}"bamboo species"` flower `color{Violet}"only once"` in their life time, generally `color{Violet}"after 50-100 years"`, produce large number of fruits and die.

`star` Another plant, `color{Violet}"𝘚𝘵𝘳𝘰𝘣𝘪𝘭𝘢𝘯𝘵𝘩𝘶𝘴 𝘬𝘶𝘯𝘵𝘩𝘪𝘢𝘯𝘢 (neelakuranji)"`, flowers once in `color{Violet}"12 years"`.

`star` This plant flowered during `color{Violet}"September-October 2006"`. Its mass flowering transformed large tracks of hilly areas in Kerala, Karnataka and Tamil Nadu into `color{Violet}"blue stretches"` and attracted a large number of tourists.



`color{Brown}"ANIMALS"` :

● Birds living in `color{Violet}"nature"` lay eggs only `color{Violet}"seasonally"`.

● However, birds in `color{Violet}"captivity"` (as in poultry farms) can be made to lay eggs `color{Violet}"throughout the year"`.

● In this case, laying eggs is not related to reproduction but is a `color{Violet}"commercial exploitation"` for human welfare.

`color{blue}"Embryogenesis"`: Embryogenesis refers to the process of `color{violet}"development of embryo"` from the `color{violet}"zygote"`.

`color{green} ✍️ color{green} mathbf("KEY CONCEPT")`

● After attainment of `color{Violet}"maturity"`, all sexually reproducing organisms exhibit events and processes that have remarkable `color{Violet}"fundamental similarity"`, even though the `color{Violet}"structures associated"` with sexual reproduction are indeed very different.

● The events of sexual reproduction though `color{Violet}"elaborate and complex"`, follow a `color{Violet}"regular sequence"`.

● Sexual reproduction is characterised by the `color{Violet}"fusion (or fertilisation)"` of the male and female gametes, the `color{Violet}"formation of zygote"` and `color{Violet}"embryogenesis"`.

● For convenience these sequential events may be grouped into `color{Violet}"three"` distinct stages namely,
the `color{Violet}"pre-fertilisation"`, `color{Violet}"fertilisation"` and the `color{Violet}"post-fertilisation"` events.