Endosperm
Endosperm is a usually triploid tissue (containing three sets of
chromosomes) produced in the seeds of most
flowering plants. It provides nutrition to the developing
embryo. It is mostly composed of
starch, though it can also contain
oils and
protein. Although usually considered triploid, the ploidy of endosperm varies widely, from diploid (2n) to 15n. [
1]
Endosperm is formed when the two sperm inside a
pollen grain reach the interior of an embryo sac or female gametophyte. One sperm fertilizes the
egg, forming a
zygote, while the other sperm usually fuses with the two polar nuclei at the center of the ovary, creating endosperm (
double fertilization). The zygote divides to form the plant embryo, while the endosperm nuclei divide to form the endosperm tissue that serves to nourish the developing embryo.
In some species (e.g. grains) the endosperm persists to the mature seed stage as a storage tissue, and in others it is absorbed during embryo development (e.g.
common bean,
Phaseolus vulgaris) and the function of storage tissue is performed by enlarged seed leaves (
cotyledons). In certain species (e.g. corn,
Zea mays) the storage function is distributed between both endosperm and the embryo. Some mature endosperm tissues store fats (e.g.
castor bean,
Ricinis communis) and others (including grains, such as wheat and corn) store mainly starches. The dust-like seeds of
orchids have no endosperm. Orchid seedlings are
mycoheterotrophic in their early development. In some other species, such as
coffee, the endosperm also does not develop.[
2] Instead the nucellus produces a nutritive tissue termed perisperm.
There are two different types of endosperm formation, the nuclear (or liquid endosperm) type, where formation of cell wall is delayed for a number of cell divisions and the cellular where cell wall formation is initiated instantly. The nuclear type is the most common one in angiosperms.
Sweet corn is picked for eating at the tender liquid endosperm stage, before cell walls have formed and the sugars have been converted to starch. The 'milk' of the
coconut is a liquid endosperm.
Cereal crops are grown for their palatable
fruit (
grains or
caryopsis), which are primarily endosperm. In the caryopsis, the thin fruit wall is fused to the seed coat. Therefore, the nutritious part of the grain is the seed and its endosperm. In some cases (e.g. wheat, rice) the endosperm is selectively retained in food processing (as in white flour), and the embryo and seed coat removed. Endosperm thus has an important role within the human diet, worldwide.
The
aleurone is a maternal tissue that is retained as part of the seed in many small grains. The aleurone functions for both storage and digestion. During germination it secretes the
amylase enzyme that breaks down endosperm starch into sugars to nourish the growing seedling.
[Endosperm Development. URL accessed on April 29, 2006.]
