● These include all the events of sexual reproduction `color{Violet}"prior to the fusion"` of gametes.

● The two main pre-fertilisation events are `color{Violet}"gametogenesis"` and `color{Violet}"gamete transfer"`.

`color{blue}"Gametogenesis"`: Gametogenesis refers to the process of `color{violet}"formation"` of the `color{violet}"two types of haploid gametes"` i.e., male and female gamete (egg).

`color{blue}"Isogametes/ Homogametes"`: Those gametes that are so `color{violet}"similar in appearance and behaviour"` that it is not possible to categorise them into male and female gametes.The fusion of such gametes id called `color{violet}"isogamy"`.

`color{blue}"Heterogametes"`: The `color{violet}"morphologically distinct gametes"` found in the majority of sexually reproducing organisms. In such organisms the male gamete is called the `color{violet}"antherozoid or sperm"` and the female gamete is called the `color{violet}"egg or ovum"`.

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

● `color{Brown}"Gametogenesis"` refers to the process of formation of the two types of gametes – `color{Violet}"male and female"`.

● Gametes are `color{Violet}"haploid"` cells.

● In some `color{Violet}"algae"` the two gametes are so `color{Violet}"similar in appearance"` that it is not possible to categorise them into male and female gametes.

● They are hence, are called `color{Violet}"homogametes (isogametes)"` and such reproduction is `color{Brown}"isogamy"`.

● However, in a majority of `color{Violet}"sexually reproducing organisms"` the gametes produced are of two morphologically distinct types (`color{Brown}"heterogametes"`).

● In such organisms the male gamete is called the `color{Violet}"antherozoid or sperm"` and the female gamete is called the `color{Violet}"egg or ovum"`.

`color{blue}"Monoecious"`: In context of a plant or an invertebrate animal, it means having `color{violet}"both the male and female"` reproductive organs in the `color{violet}"same individual"`; called `color{violet}"homothallic"` in algae and `color{violet}"hermaphrodite"`, bisexual in animals.

`color{blue}"Dioecious"`: In context of a plant or an invertebrate animal, it means having `color{violet}"both the male and female"` reproductive organs in `color{violet}"different individuals"`.

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

● Sexual reproduction in organisms generally involves the `color{Violet}"fusion of gametes"` from two different individuals, though this is not always true.

`color{Brown}"PLANTS"`

● Plants may have both `color{Violet}"male and female"` reproductive structures in the same plant (`color{Violet}"bisexual"`) or on different plants (`color{Violet}"unisexual"`).

● In several fungi and plants, terms such as `color{Brown}"homothallic and monoecious"` are used to denote the `color{Violet}"bisexual"` condition and `color{Brown}"heterothallic and dioecious"` are the terms used to describe `color{Violet}"unisexual"` condition.

● In flowering plants, the unisexual male flower is `color{Violet}"staminate"`, i.e., bearing stamens, while the female is `color{Violet}"pistillate"` or bearing pistils.



● In some `color{Violet}"flowering plants"`, both male and female flowers may be present on the same individual (`color{Violet}"monoecious"`) or on separate individuals (`color{Violet}"dioecious"`).



● Some examples of monoecious plants are `color{Violet}"cucurbits and coconuts"` and of dioecious plants are `color{Violet}"papaya and date palm"`.

`color{Brown}"ANIMALS"`

● Earthworms, sponge, tapeworm and leech, typical are examples of `color{Violet}"bisexual animals"` that possess both male and female reproductive organs, and are called `color{Brown}"hermaphrodites"`.



● Cockroach is an example of a `color{Violet}"unisexual species"`.

● Gametes in all `color{Violet}"heterogametic species"` are of two types namely, `color{Violet}"male and female"`.

● Gametes are `color{Violet}"haploid"`, though the `color{Violet}"parent plant"` body from which they arise may be either `color{Violet}"haploid or diploid"`.

● A haploid parent produces gametes by `color{Violet}"mitotic division"`.

● Several organisms belonging to monera, fungi, algae and bryophytes have `color{Violet}"haploid plant body"`.



● In plants belonging to `color{Violet}"pteridophytes, gymnosperms, angiosperms"` and most of the animals including human beings, the parental body is `color{Violet}"diploid"`.



● It is obvious that `color{Violet}"meiosis"`, the `color{Violet}"reduction division"`, has to occur if a diploid body has to produce haploid gametes.

● In diploid organisms, specialised cells called `color{Violet}"meiocytes"` (`color{Violet}"gamete mother cell"`) undergo meiosis.

● At the end of meiosis, only `color{Violet}"one set of chromosomes"` gets incorporated into each gamete.

● After their formation, male and female gametes must be `color{Violet}"physically brought together"` to facilitate fusion (`color{Violet}"fertilisation"`).

● In a majority of organisms, male gamete is `color{Violet}"motile"` and the female gamete is `color{Violet}"stationary"`.

● Exceptions are a `color{Violet}"few fungi and algae"` in which both types of gametes are motile.

● There is a `color{Violet}"need for a medium"` through which the male gametes move.

● In several simple plants like algae, bryophytes and pteridophytes, `color{Violet}"water is the medium"` through which this gamete transfer takes place.



● A large number of the male gametes, however, `color{Violet}"fail to reach"` the female gametes.

● To `color{Violet}"compensate this loss"` of male gametes during transport, the number of male gametes produced is `color{Violet}"several thousand times"` the number of female gametes produced.

● In seed plants, pollen grains are the `color{Violet}"carriers of male gametes"` and `color{Violet}"ovule have the egg"`.

● Pollen grains produced in `color{Violet}"anthers"` therefore, have to be `color{Violet}"transferred to the stigma"` before it can lead to fertilisation .

● In bisexual, `color{Violet}"self-fertilizing/ self-pollinating plants"`, e.g., peas, transfer of pollen grains to the stigma is `color{Violet}"relatively easy"` as anthers and stigma are located `color{Violet}"close to each other"`; pollen grains soon after they are shed, come in contact with the stigma.



● But in `color{Violet}"cross pollinating plants"` (including dioecious plants), a specialised event called `color{Violet}"pollination"` facilitates transfer of pollen grains to the stigma.



● Pollen grains `color{Violet}"germinate"` on the stigma and the `color{Violet}"pollen tubes"` carrying the male gametes reach the `color{Violet}"ovule"` and discharge male gametes near the egg.



● In `color{Violet}"dioecious"` animals, since male and female gametes are formed in `color{Violet}"different individuals"`, the organism must evolve a `color{Violet}"special mechanism"` for gamete transfer.

● `color{Violet}"Successful transfer"` and `color{Violet}"coming together"` of gametes is essential for the most critical event in sexual reproduction, the `color{Violet}"fertilisation"`.