Describe in brief the various special modes of reproduction? Mention their economic significance?
Special modes of reproduction: 1. Apomixis (Agamospermy): It is a modified form of reproduction in which seeds are formed without fusion of gametes. It includes the process whereby a diploid cell of nucellus develops into an embryo, giving a diploid seed with a genetic constitution identical to the parent. The organism that reproduces by apomixis is called apomict. 2. Parthenocarpy: In certain plants, the fruits are formed without the act of fertilisation. Such fruits are called parthenocarpic fruits and the phenomenon is known as parthenocarpy. Parthenocarpic fruits are either seedless or contain empty or non-viable seeds. In these fruits, the stimulus (“seed factor”) for fruit growth is provided by the tissue of the ovary wall itself. Seedless varieties of grapes and oranges contain those of the seeded varieties. Most commonly cultivated varieties of banana and pineapple are parthenocarpic. 3. Polyembryony: It is the formation of more than one embryo in the seed. Onion, groundnut, mango, lemon and oranges are some of the examples. Polyembryony is very common among conifers. Sporophytic budding: It may occur in the nucellus and integuments, resulting in the development of an embryo. Examples: Orange, mango, onion, etc. 5. Micropropagation: In this method, tissue and organ culture is utilised. Tissue or organs grow on suitable medium containing hormones. Tissue proliferates to form a callus. From this callus, arise new plantlets. Each plantlet, when transferred to pot or soil, produces a new plant. Thus by this method, an indefinite number of plants can be raised from a small mass of partial tissue. Plant tissue culture. It also helps in propagating plants to a great extent.
