Describe polyembryony and parthenocarpy

In several genera of angiosperms more than one viable embryo in the seed has been observed. It is more frequent in Gymnosperms than in angiosperms. This condition arises in various ways:

(i) The oospore produces more than one embryo during development as a result of splitting. This is known as cleavage polyembryony and is very common in gymnosperms (e.g. Pinus) and angiosperms e.g. Allium, citrus and some orchids.

(ii) An ovule may contain more than one functional megaspore producing as many embryo sacs and oospheres which on fertilization give rise to more than one embryo in the seed.

(ii) In addition to the normal embryo produced from a fertilized oosphere, embryo may also be formed from the synergids (e.g. Dandelion) or rarely the antipodals with or without fertilization.

(iv) Additional embryos may be produced as a result of sporophytic budding by the division of the cells of the nucellus (e.g. lemon) or integument (e.g. onion).

Parthenocarpy:

As a rule the development of the ovule into seeds and the enlargement of the ovary into fruit follow only after pollination and fertilization has occurred. In most species of flowering plants, the failure of pollination results in the failure of seed and fruit formation. This usually happens in mature of the pollinating insects are not active or destruction of pollen occurs. In many kinds of fruit trees, if heavy rains fall during the time when pollen is maturing the pollen grains are washed out of the flowers, pollination does not occur and as a consequence few or no flowers set fruits. In some plants fruits are formed even in the abundance of pollination and fertilization. Such fruits are always seed less. These are succulent and more desirable than normal seed containing fruits. They are said to be partheocarpic while the phenomenon concerned in their formation is termed Parthenocarpy. Parthenocarpy is of two types:

(a) Induced parthenocarpy: When seed less fruits are produced by spraying the stigmas of flowers with ether or water extract of pollen grains or by injecting such growth promoting substances as in indole acetic acid and naphthalene acetic acid into ovaries of plants.

(b) Natural parthenocarpy: When seed less fruits are produced without any special treatment from the ovaries in the absence of pollination and fertilization. Familiar examples are the naval oranges, grapes, bananas and pine apples.