Flowers, seeds and fruit

At least 25,000 species of orchid have been named, and some of them get pollinated by tricking wasps.

The parts of a flower

A flower is a shoot built for reproduction. On the outside are green sepals, which protected the bud, and inside them the petals, often coloured to attract animals. The male parts are the stamens: each has a stalk, the filament, topped by an anther that makes pollen. The female part in the middle is the carpel. Its sticky tip, the stigma, catches pollen; a stalk called the style leads down to the ovary, which holds one or more ovules.

Getting pollen to the stigma

Pollination is the transfer of pollen from an anther to a stigma. In self-pollination the pollen lands on a stigma of the same flower or the same plant. In cross-pollination it reaches a flower on a different plant of the same species, which mixes genes and gives more varied offspring. Many plants make self-pollination hard: their pollen and stigma ripen at different times, they carry separate male and female flowers (cucumber), or male and female flowers grow on different plants. Gregor Mendel used both self- and cross-pollination of garden peas in his studies of heredity.

Most pollinators are animals, and flowers are shaped for them. Bees cannot see red, so bee flowers tend to be blue or yellow, often with nectar guides, markings that bees can see and people cannot. Fly-pollinated flowers such as the corpse flower smell of rotting meat and have dull brown or purple colours. Moths visit pale, flat flowers in the evening; the yucca and the yucca moth depend on each other. Bats pollinate large, pale, strong-smelling night flowers such as agave. Hummingbirds visit bright, curved, tubular flowers that often have no scent. Grasses, maples, oaks and most conifers use the wind instead: small green flowers or cones, no nectar, huge amounts of light pollen. Some orchids offer nothing at all and trick their pollinators, and the hammer orchid even looks and smells like a female wasp.

Double fertilization and the seed

After landing on the stigma, a pollen grain grows a tube down through the style to an ovule, and two sperm cells travel down it. In flowering plants both of them are used. One fertilizes the egg and makes a diploid (2n) zygote that becomes the embryo. The other joins two polar nuclei and makes a triploid (3n) cell that grows into the endosperm, the food store. This is double fertilization. The fertilized ovule becomes the seed, and the ovary around it becomes the fruit.

A seed contains a seed coat, one or two cotyledons (seed leaves), often an endosperm, and an embryo. The embryo’s radicle becomes the first root and its plumule the first shoot and leaves. Dicots have two cotyledons; the two halves of a peanut and of a split pea are cotyledons packed with food. Monocots such as corn and wheat have one, called the scutellum, and keep their food in a large endosperm. Many seeds then wait in dormancy, for months, years or longer, until water, light, cold, heat or even fire tells them it is time to germinate.

Fruit and getting away

To a botanist a fruit is a ripened ovary, so a tomato, a peanut pod and a whole corn kernel are all fruits. A simple fruit grows from one ovary (beans, nuts). An aggregate fruit grows from several carpels of one flower (raspberry). A multiple fruit grows from a cluster of flowers (pineapple). An accessory fruit is mostly made of another flower part, such as the receptacle in a strawberry or apple. A fruit has one purpose, dispersal: dandelion seeds float on the wind, coconuts float across the sea, and squirrels bury acorns and forget some of them.