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real banana seeds

Where, and under what circumstances, water penetrates the inner integument is unknown. In many non-banana species it happens via water gaps [9] . In bananas, water gaps might develop around the micropylar plug [10] , or in the area of the chalazal mass, where the inner integument is discontinuous, and parenchymatic tissue separates the outer and inner chambers [2] .

Water must enter the chamber containing the embryo and endosperm before germination can occur. Experiments have shown that the seed’s outer integument is leaky since rupturing the outer integument of Musa acuminata seeds by sandpapering did not increase the absorption of water. During imbibition, the weight of the sandpapered seeds increased by the same proportion (33%) as the one of intact seeds (see graph) [8] .
In their natural habitat, humid tropical forests, banana seeds may lie viable for years, often germinating at the same time and in large numbers when soil is disturbed or vegetation removed [17] . The higher temperature of exposed soil in gaps in the forest suggests that temperature is a cue for banana seeds to germinate. Under greenhouse conditions, the germination of Musa balbisiana seeds was greatest when temperature fluctuations were large [18] .

It is likely that there are different types of dormancy among Musa species, with contributions from the seed coat and the embryo. The necessary experimentation to clearly define the roles of the coat, embryo and other tissues in dormancy have not been done, but that banana seeds can become dormant appears beyond doubt [16] .
The seed, like the one of the wild species, Musa balbisiana, has an extremely durable, wrinkled coat, the only opening of which is sealed by a shortened conical micropylar plug, also called an operculum (see figure). The seed coat consists of a thick hardened, but leaky, outer integument, and a thin inner integument, on the inner surface of which is a thick cuticle. Seeds of wild bananas are usually less than 1 cm in diameter [1] .
While size, colour and shape varied, a study of 20 species of wild and edible bananas found this basic seed structure and anatomy in all of them [5] .
A seed has one living component, the embryo. To germinate, not only does the embryo have to come into contact with water, the seed also has to be non-dormant.
In dry mature seeds, the hard, leaky outer layer of the coat (the outer integument) acts like a cage, while the thin inner layer of the coat (the inner integument), which is lined with a cuticle, is impermeable to water and provides an almost complete envelope surrounding the embryo and endosperm. Only wild species of bananas — and under certain circumstances cultivars that have residual fertility — produce seeds.

The basic external requirements for germination are water, oxygen and an appropriate temperature. The germination of a seed begins with water uptake (imbibition), followed by embryo expansion along its axis through cell elongation, and ends with the breaking-through of the seed coat by the embryo root (radicle), then shoot (plumule). In the banana seed, the endosperm disappears within two weeks and within three weeks, the seedling has a lateral root system, and a prominent shoot [6] , in which the first leaves have elongated beyond the cotyledonary sheath [7] . By the fourth week, the seedling is dependent on the root system for inorganic nutrition [6] .

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