Instant Notes: Plant Biology

Section D – Reproductive anatomy

D1 The flower

Flowersare unique to flowering plants, normally developing on vegetative

shoots (Topic H1). A typical flower is hermaphrodite, i.e. bearing fertile organs

of both sexes, and is pollinated by insects (Fig. 1). The stalk that bears the flower

General structure
of the flower

Key Notes

In a typical flowering shoot there is a receptacle at the top of a stem with

four whorls of floral organs: sepals, petals, stamens and carpels. Sepals

are usually green and protective, petals often brightly colored and

attractive to animals. The stamens have a filament with the pollen-

bearing anthers on it. The carpel consists of an ovary enclosing the

ovules, a style and a stigma with a receptive surface for pollen grains. In

some plants one or more whorls may be absent, e.g. in unisexual flowers

and reduced flowers of wind-pollinated plants.

Sepals resemble simple leaves. Petals have a specialized epidermis, often

with pigmented vacuoles and air-filled reflective mesophyll. The

pigments are mainly flavonoids with some betalains and carotenoids.

Insects have different color sensitivity from us and the colors of petals are

different and more varied for insects.

The filament bears four or two anthers. The anther wall has three layers

and splits (dehisces) to release pollen. Some plants have specialized

dehiscence such as only releasing pollen when insects vibrate their wings;

orchids have sticky pollen masses known as pollinia. The filaments may

be long and thin or flat and some are sensitive to insect contact.

The ovaries may be separate or fused, and on the surface, i.e. superior, or

buried in the receptacle, inferior. There may or may not be a style. In

animal-pollinated plants the stigma is club-shaped or lobed and may by

dry or sticky; in wind-pollinated plants it is usually feathery with a large

surface area. The ovaries contain one to many ovules. The ovules usually

have two integuments and a nucellus surrounding the embryo sac, with a

tiny gap, the micropyle, for pollen tube entry.

Nectar is mainly a sugar solution of varying concentration with small

quantities of other substances. It is secreted at the base of the petals or

elsewhere in the flower. Many flowers have no nectar and use pollen as a

food reward, or more rarely oil or resin.

Related topics Pollen and ovules (D2) Self incompatibility (H3)

The seed (D3) Amino acid, lipid, polysaccharide

Physiology of floral initiation and and secondary plant metabolism

development (H1) (J5)

Breeding systems (H2)

Stamens

General structure of

the flower

Sepal and petal

structure

Carpels

Nectar