|Where do seeds come from? I know flowers form from
seeds but how do seeds form?
|Question Date: 2004-05-12|
A seed is a plant embryo, plus nutrient
tissues to feed the embryo, packaged in a
protective shell. Since it's an embryo, it's the
product of sexual reproduction.
Here's how it works.
When a seed
germinates, a plant grows out of it. It's only
partially true to say that flowers form from seeds
because, actually, the whole plant forms from the
seed. Even many non-flowering plants like pine
trees come from seeds. After the plant matures, it
reproduces. For flowering plants, that means they
make flowers to attract pollinators. Pollen is
delivered to flowers by bees, wind, or a number of
other "vectors." Sperm cells leave the pollen
and fertilize the eggs, which are found deep
inside the flower. The outer parts of the flower
(the parts we think are pretty) then die, but the
inner parts that contain the fertilized eggs
develop into fruit. All fruits contain seeds (so
tomatoes and zucchinis are fruits, not
vegetables). The fruit's job is usually either to
provide food for the baby plant or to tempt an
animal to eat the fruit, spreading the seeds in
the process. Once the seeds are out of the fruit
and into the ground, they grow into new plants.
I hope that answers your question!
I will here ignore the (potentially
complicated) process by which a pollen grain gets
to an ovule; it may be by wind or the pollen
grain may travel via a pollinating organism.
In any case, when the sperm reaches the ovule it
fertilizes the egg to form a zygote which develops
into a young embryo. As this proceeds, the embryo
and its associated nutritive cells enlarge, and so
does the protective integument.
As the embryo plus nutritive reserves reach
maturity, the integument generally becomes hard
and resistant and is then refereed to as a seed
coat. The whole structure (embryo, food store &
integument) is then referred to as a seed,
which may be dispersed by wind or animals, thus
spreading the genotype of the plant
over the surface of the Earth. The same
process, with certain variants, occurs in other
groups of plants that bear seeds (e.g., pine
trees, cycads, etc.)
Seed plants (not just flowering plants, but
not all plants) have organs called ovules where
they are going to produce seeds. The ovules
are fertilized by pollen, and then mature
and grow to become seeds. In a flowering plant,
the ovules are contained within the ovary, which
is a bulbous structure at the base of the flower.
In a seed plant that is not a flowering plant
(such as a conifer), the ovules are located inside
of the scales of the cones.
Consider the example of an apple. The fruit is
the ovary, which has grown and enlarged after
fertilization of the ovules (which have become
seeds). The flower, what is left of it (the petals
have all fallen off), is still on the apple - on
the end of the apple opposite the stem.
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