The range of seed size is extreme more than nine orders of magnitude. The largest known seed is that of the double coconut (Lodoicea maldivica); the seed and fruit together weigh as much as 27 kilograms. At the other end of the scale, the dustlike seeds of some orchids, begonias, and rushes weigh only about 5 milligrams per seed. It is thought that the size of seed displayed by each species represents a compromise between the requirements for dispersal (which would favor smaller seeds that can be borne on wind or picked up by animals) and the requirements for establishment of the seedling (which would favor larger seeds that can adhere to a growth medium).
The chemical composition of seeds varies widely among species. In addition to the normal compounds found in all plant tissues, seeds contain unique food reserves that are used to support early seedling growth. About 90 percent of plant species use lipids (fats and oils) as their main seed reserves. The cotyledons of soybeans and peanuts are rich in oil, whereas in other legumes such as peas and beans, starch is the reserve material. Sixty-four percent of the weight of a castor bean is derived from the oil stored in the endosperm. In seeds of cereal crops, the endosperm stores much starch, in corn it can be up to 80 percent of the weight of the seed. All seeds, particularly legumes, also store protein as a reserve substance.
See also: Structure, Dispersal, Dormancy
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