Biology, asked by sivamthapa64321, 1 year ago

Structure in bajra flower favouring cross pollination

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Answered by yoyobaba
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1. Imperfect (or unisexual) flowers and monoecy.

The separation of the male sexual part (stamen) and the female part (pistil) into two separate flowers eliminates intrafloral pollination and favors outcrossing. In monoecy (adj. monoecious), or monoecism, both imperfect flowers are borne on the same plant.

In many cucurbits (e.g. pumpkin and squash, melons, and gourds) the unpollinated pistillate flower looks like a miniature fruit with attached showy petals. 

In corn or maize, a monoecious crop plant, the pistillate flowers are found in the tassel, a male inflorescence. The tassel emerges upward from the terminal portion or tip of the stem (culm). The staminate flowers are found on a modified, lateral (axillary) shoot consisting of an ear (an inflorescence) enclosed in a protective husk. Consequently, corn is about 95% cross-pollinated and only 5% self-pollinated (Poehlman 1977).

2. Imperfect flowers and dioecy.

The natural occurence of individual plants bearing either staminate or pistillate flowers ensures cross-pollination. This phenomenon is called dioecy (adj. dioecious). 

However, there are plant populations which exhibit modifications of dioecy. In gynodioecy, some individual plants bear pistillate flowers exclusively, while others have perfect flowers; in androdioecy, some plants have staminate flowers only while others have perfect flowers; in trioecy (e.g. papaya), there are three plant variants: some individual plants may have staminate flowers only, some with pistillate flowers only, and others with perfect flowers (Simpson 2010). 

3. Dichogamy.

Opposite of homogamy, the stamen and the pistil mature at different periods. There are two main types: protandry and protogyny. 

In protandry, the stamens or anthers develop ahead and the pollen grains mature and are shed before the pistils or the stigma become mature and receptive. This type of pollination occurs in many members of the family Asteraceae (Sunflower family), Apiaceae (Carrot family), and Campanulaceae (Bluebell family). In protogyny, such as in some members of Chenopodiaceae (e.g. Suaeda; Simpson 2010) and avocado (Allard 1960), the pistils or stigma mature ahead of the stamens or anthers. 

5. Chasmogamy. 

Opposite of cleistogamy, pollen is shed, the stigma becomes receptive, and pollination occurs when the flower opens (Contreras 2007). The opening of the flower exposes the stigma to pollen from other flowers.

6. Hercogamy (or herkogamy).

This is the spatial separation of the male (stamens) and female (stigma) sexual organs within a flower. In one type  of hercogamy called heterostyly, the heights of stigmas relative to the stamens vary from flower to flower.  In enantiostyly, or enantiosmorphy, there are “left-handed” flowers in which the styles curve to the left and “right-handed” flowers with styles that curve to the right. In both cases, at least one stamen usually curves towards the direction opposite to that of the style. 

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