Describe self-pollination, cross-pollination and often cross-pollination. Discuss different mechanisms for facilitating self-pollination and cross-pollination.

Self-Pollination

Self-pollination, also known as autogamy, is the transfer of pollen from the anther to the stigma of the same flower or another flower on the same plant. For self-pollination to occur, the anthers and stigmas must mature simultaneously and be positioned appropriately. Plants exhibiting self-pollination often have closed floral structures to prevent external agents from interfering.

Mechanisms Facilitating Self-Pollination:

• Cleistogamy: This is a type of self-pollination where flowers remain closed and pollination occurs within the bud. The resulting seeds are genetically identical to the parent plant. Example: Viola (common violet), Oxalis.
• Homogamy: Here, flowers open, but pollination occurs before the anthers and stigmas fully separate. This minimizes the chances of cross-pollination.
• Anther and Stigma Proximity: Some plants have anthers and stigmas positioned close together, facilitating self-pollination.
• Anther and Stigma Maturation Timing: Simultaneous maturation of anthers and stigma ensures pollen is available for self-pollination.

Cross-Pollination

Cross-pollination, or allogamy, is the transfer of pollen from the anther of one plant to the stigma of a different plant of the same species. This process promotes genetic recombination, leading to greater diversity in offspring. Cross-pollination requires agents like wind, water, insects, or animals for pollen dispersal.

Mechanisms Facilitating Cross-Pollination:

• Wind Pollination (Anemophily): Plants relying on wind pollination typically produce copious amounts of lightweight, non-sticky pollen. They often have inconspicuous flowers with feathery stigmas to capture windborne pollen. Example: Grasses, Betula (birch).
• Water Pollination (Hydrophily): Less common, this involves pollen transfer via water. Pollen can be released freely into the water or carried attached to the plant. Example: Vallisneria (tape grass).
• Insect Pollination (Entomophily): Many flowering plants rely on insects like bees, butterflies, moths, and flies for pollination. These plants often have brightly colored petals, nectar rewards, and fragrances to attract pollinators. Specific adaptations, like specialized flower shapes, ensure pollen transfer.
• Bird Pollination (Ornithophily): Birds, particularly hummingbirds and sunbirds, are important pollinators for some plants. These plants often have tubular flowers with red or orange coloration and copious nectar.
• Bat Pollination (Chiropterophily): Some plants are pollinated by bats, which are attracted to night-blooming flowers with strong fragrances and abundant nectar.

Often Cross-Pollination

Often cross-pollination, also referred to as pseudo-self-pollination or facultative xenogamy, describes a situation where plants have mechanisms that primarily facilitate cross-pollination but occasionally experience self-pollination. This provides a degree of genetic mixing while ensuring reproduction even in the absence of pollinators.

Mechanisms leading to Often Cross-Pollination:

• Self-incompatibility: This is a genetic mechanism that prevents self-pollination. The plant recognizes its own pollen and blocks its germination or fertilization. However, mutations can sometimes overcome this barrier, leading to occasional self-pollination.
• Heterostyly: Plants with heterostyly have different styles and stamen lengths in different floral morphs (e.g., "pin" and "thrum" forms in Primula). This mechanism reduces self-pollination, but rare events can still lead to self-fertilization.
• Protandry/Protogyny: Protandry is when anthers mature before stigmas (leading to delayed self-pollination), and protogyny is when stigmas mature before anthers. These strategies promote cross-pollination but can sometimes result in self-pollination if the timing is altered.

Pollination Type	Advantages	Disadvantages	Examples
Self-Pollination	Ensures reproduction even with limited pollinators, rapid population growth	Reduced genetic diversity, increased susceptibility to disease	Viola, Oxalis
Cross-Pollination	Increased genetic diversity, adaptability, reduced inbreeding depression	Requires pollinators, less reliable	Grasses, Primula
Often Cross-Pollination	Combines benefits of both self and cross-pollination	Complex genetic mechanisms	Primula (with heterostyly)