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Banana flowers


Banana inflorescence at a glance
Basal female flowers becoming fruits (top right) and distal male flowers revealed by lifting bract (bottom).
Basal female flowers becoming fruits (top right) and distal male flowers revealed by lifting bract (bottom).

Most flowers have both functional female and male organs. While a banana flower also has both female and male organs, the relative development of these organs will determine whether the flower is ultimately female, and develop into fruit, or male, and only produce pollen[1]. This is true of both wild species of banana and the cultivars that have been domesticated for edibility, except that some wild species also produce hermaphrodite flowers, that is flowers which have female and male organs that are both functional.

Morphology

The female organs are the ovary, style and stigma. These together make up the pistil, also called a carpel. In a banana flower, three pistils fuse producing a tri-pistillate ovary, style and stigma. This is the female part of the flower, in botany called the gynoecium (from the Greek ‘gyne’ = women’s and ‘oikos’ = house).

The male organs are the filament and anther, which together form a stamen. A banana flower typically has five stamens. These collectively form the male part of the flower, the androecium (from the Greek ‘aner, andr-‘ = man, and ‘oikion’ = house).

The style, stigma and male parts of the banana flower are enveloped within a tubular structure formed by fusion of five petal-like tepals, with a sixth tepal remaining free.

Female and male flowers have nectaries located at the top end of the ovary, near the base of the filaments and style. Female flowers produce a greater quantity of nectar, with a higher sugar concentration than that of male flowers.

Types of flowers

Female flower

Female flowers typically have a well-developed gynoecium and a non-functioning androecium, with less-well to poorly-developed male organs and no pollen. Sometimes the male organs are absent. 

In wild species, the fruit will contain seeds if pollinated. In edible bananas, the ovary develops into a seedless fruit by parthenocarpy (without being pollinated).

Male flower

In male flowers the gynoecium is much smaller than in female flowers and there is a well-developed androecium that in wild species produces viable pollen. In edible bananas, the amount of pollen is reduced or absent. Male flowers usually fall to the ground a short time after flowering

Hermaphrodite flowers

In some wild species, some basal fruit-forming flowers have a functional gynoecium and androecium and can self-fertilise. This can occur before bract opening if the stigma and anthers are aligned. These flowers are hermaphrodite or perfect and can effectively prevent out-crossing. Hermaphrodite flowers have not been described in edible bananas.

Other types of flowers

In some edible bananas, there may be flowers at the point of transition from the basal female to the distal male portion of the inflorescence that do not produce fruit and have a small ovary although it is larger than in male flowers. These flowers may retain some features of fruit-forming flowers, such as remaining attached to the peduncle. Such flowers are called neuter or intermediate. They are also described in some wild species, where they can be neuter or functionally male.

Inflorescence

Banana flowers are arranged in clusters on the peduncle of the inflorescence. The clusters are called hands, and each hand is enfolded by a bract that lifts at flowering. Within the hand, the flowers are usually formed in two rows in a type of flower formation called a cincinnus[2]. Each flower is connected to the peduncle by a shortened stem called the pedicel.

The female flowers are near the base of the peduncle and the male fowers at the distal end of the peduncle.

Pollination

Flowers provide the structure for sexual reproduction in the banana plant and produce seeds. Sexual reproduction in bananas occurs when pollen produced by the anther of a male flower fertilises the ovule in a female flower to produce a viable embryo.

First, pollen needs to be transferred to the stigma. Since the female flowers open before the male flowers on the same inflorescence,  more than one inflorescence and a pollinator to collect and deliver the pollen are essential. 

Second, once on the stigma, the pollen grains need to be ‘recognised’ in order to germinate. The tube emerging from a germinated pollen grain responds to chemical signals that guide it down the canal in the centre of the style to reach one of the three locules where ovules are located. Further guidance is needed to get a pollen tube to an available ovule to form a viable embryo.

The fertilised ovule containing the viable embryo develops into a seed. This in turn stimulates pulp development around the seeds in the ovary, resulting in a seed-bearing banana fruit. A banana fruit in a wild species might contain up to 250 seeds.

Pollinators

The nature of the banana inflorescence is that female flowers are separated in space and time from male flowers. In this case, pollinators are essential for seed production.

Since the tepals are not colourful and nectar is abundant, the main pollinators are bats and birds. Banana flowers are visited by numerous species of insects and some animals but these are not involved in pollination.

Edible bananas

In edible bananas, sexual reproduction is rarely successful, with very few if any seeds produced as a consequence of pollination. This failure is multifaceted, due to a greater or lesser extent to a lack of viable pollen, disruption of the pollen pathway through the gynoecium in the female flower and a lack of viable ovules.

Instead the fruit of edible bananas develop through vegetative parthenocarpy, with the pulp developing autonomously from tissues on the ovary wall of the female flower without the need for pollination. The parthenocarpic fruit of the edible banana is the banana’s unit of commerce.

References

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