Instant Notes: Plant Biology

Section D – Reproductive anatomy

D3 The seed

Definition The ovule matures into a seed before it is dispersed from the parent plant. In
flowering plants, the seeds are contained within fruitsat maturity, although in
other seed plants they are exposed. Flowering plant seeds consist of a seed coat,
endosperm and the embryo. They are enormously variable in size, shape and
structure, from coconuts weighing several kilograms to the microscopic seeds of
orchids.

The seed coat As the seed matures, the integuments of the ovule develop into the seed coat or
testa. The structure of the testa is highly variable and it may be formed from one
or both integuments and sometimes the nucellus as well (Topic D1). The testa
hardens with the deposition of cutin or lignin and phenols and, in many plants,
it is extremely resistant. Some can survive a passage through the gut of a verte-
brate and a few, such as some eucalyptus seeds, can survive a fire; many can lie
dormant (Topic H4). In some seeds the outer integument or part of it develops
into a fleshy coat, or aril, attractive to animals, with a hard testa inside. Plants
with dry indehiscent fruits may have almost no seed coat at all, its function
being replaced by the fruit, as in grasses.

Key Notes

The seed is the mature ovule and consists of a seed coat, endosperm and

embryo. In angiosperms it is enclosed within a fruit.

Known as the testa, this is normally formed from one or both

integuments. Frequently it is tough and survives unfavorable conditions;

a few species have a fleshy outer integument.

This is normally triploid and the cells start dividing immediately after

fertilization, sometimes forming an acellular mass first. It is the food store

of the seed and may be rich in carbohydrate, protein or lipid. It remains

as the main part of the seed at maturity in some plants.

After the first transverse cell division the upper cell forms the shoot,

cotyledons and root, the lower cell a suspensor of no clear function. Most

flowering plants have two cotyledons but some have one and grasses

have an extra sheathing coleoptile. Some mature seeds, such as beans, are

almost entirely filled with the embryo. On germination the cotyledons

may remain in the seed and wither with shoot growth, or become the

first leaves.

Related topics Pollen and ovules (D2) Seed ecology (L2)

Fruits (D4) Seed development, dormancy and

germination (H4)

Definition

The seed coat

The endosperm

The embryo