Seeds have several advantages as a means of propagation:
• they are often produced in large numbers so that the plants regenerated from them are individually inexpensive;
• many may usually be stored for long periods without loss of viability;
• they are easily distributed;
• most often plants grown from seed are without most of the pests and diseases which may have afflicted their parents.
For many agricultural and horticultural purposes it is desirable to cultivate clones or populations of plants which are practically identical. However, the seeds of many plants typically produce plants which differ genetically, and to obtain seeds which will give uniform offspring is either very difficult, or impossible in practical terms. Genetically uniform populations of plants can result from seeds in three ways:
• from inbred (homozygous) lines which can be obtained in self-fertile (autogamous) species. Examples of autogamous crops are wheat, barley, rice and tobacco.
• from F1 seeds produced by crossing two homozygous parents. Besides being uniform, F1 plants may also display hybrid vigour. F1 seeds of many flower producing ornamentals and vegetables are now available, but due to high production costs, they are expensive.
• from apomictic seedlings. In a few genera, plants that are genotypically identical to their parents are produced by apomixis. Seeds are formed without fertilisation and their embryos develop by one of several asexual processes that ensure that the new plants are genetically identical to the female parent (i.e. they have been vegetatively reproduced) (reviewed by Van Dijk and Van Damme, 2000).
Some plants do not produce viable seeds, or do so only after a long juvenile period. Alternatively, to grow plants from seed may not provide a practical method of making new field plantings. In such instances vegetative propagation is the only means of perpetuating and multiplying a unique individual with desirable characteristics.
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