A seed is a small embryonic plant enclosed in
a covering called the seed coat, usually with
some stored food. It is the product of the ripened
ovule of gymnosperm and angiosperm plants which
occurs after fertilization and some growth within
the motherplant. The formation of the seed completes
the process of reproduction in seed plants, with
the embryo developed from the zygote and the seed
coat from the integuments of the ovule.
This process starts with double
fertilization in angiosperms and it involves the
fusion of the egg and sperm nuclei into a zygote.
The second part of this process is the fusion
of the polar nuclei with a second sperm cell nucleus,
thus forming a primary endosperm. Right after
fertilization the zygote is mostly inactive but
the primary endosperm divides rapidly to form
the endosperm tissue. This tissue becomes the
food that the young plant will consume until the
roots have developed after germination or it develops
into a hard seed coat. The seed, which is an embryo
with two points of growth is enclosed in a seed
coat with some food reserves. In gymnosperms the
two sperm cells transferred from the pollen do
not develop seed by double fertilization but instead
only one sperm fertilizes the egg while the other
is not used. The seed is composed of the embryo
and tissue from the mother plant, which also form
a cone around the seed in coniferous plants like
Pine and Spruce. |