Flowers: Pollination, Fertilization, Seeds and Fruits

Flowers are for making seeds

A flower can look like decoration, but every part of it has a job in sexual reproduction. Its colours, scents, shapes and even its timing are designed to do one thing: get pollen from one flower to another and turn ovules into seeds. In this lesson you'll follow that journey, from a bee landing on an apple blossom in May to an apple full of seeds in September.

Parts of a flower

From the outside in:

• Sepals (together called the calyx): usually green, leaf-like parts that protect the flower bud.
• Petals (together, the corolla): often colourful and scented, to attract pollinators.
• Stamens (male): each has an anther, where pollen grains are made, on a thin filament. Each pollen grain contains sperm cells.
• Pistil (female), in the centre: a sticky stigma that catches pollen, a style that holds the stigma up, and the ovary at the base, containing one or more ovules. Each ovule contains an egg cell.

All the parts sit on the receptacle, the swollen tip of the flower stalk.

A flower with all four kinds of parts is complete. A flower with both stamens and pistil is perfect. Some plants have separate male and female flowers. Corn is monoecious: the tassel at the top has male flowers, and the ear has female flowers whose long styles are the silks. Willows and trembling aspens are dioecious: each tree is either male or female.

Pollination: getting pollen to the stigma

Pollination is the transfer of pollen from an anther to a stigma. Self-pollination uses pollen from the same flower or plant; cross-pollination uses pollen from another plant of the same species, which mixes genes and produces more genetic variety in the offspring.

Wind-pollinated flowers (grasses, corn, wheat, birches, oaks, ragweed):

• small, often green flowers with no scent or nectar;
• anthers hang out in the wind; stigmas are large and feathery to catch pollen;
• huge amounts of tiny, light, smooth pollen (which is why ragweed and grass pollen cause hay fever).

Animal-pollinated flowers (apples, blueberries, clover, sunflowers, trilliums):

• bright petals, scent and sugary nectar as a reward;
• sticky or spiky pollen that clings to insects' bodies;
• shapes that fit their pollinator: bees see ultraviolet light and are guided by UV markings invisible to us; the ruby-throated hummingbird visits red, tube-shaped flowers such as the cardinal flower.

Many Canadian crops, including blueberries, apples, cherries and squash, depend on insect pollinators. Wild bees, honeybees, flies, beetles, moths and butterflies all contribute.

Fertilization: two sperm, two jobs

1. A pollen grain lands on a stigma of the right species and germinates: it grows a pollen tube down through the style toward an ovule.
2. Two sperm cells travel down the pollen tube and enter the ovule.
3. Double fertilization:

• Sperm 1 fuses with the egg to form a zygote (2n), which develops into the embryo.
• Sperm 2 fuses with two polar nuclei in the ovule to form the endosperm (3n), the food supply for the embryo.

Double fertilization is a hallmark of flowering plants. Because each ovule needs its own pollen grain, a corn ear with 500 kernels needed at least 500 pollen grains, one for each silk.

From flower to fruit and seed

After fertilization, the flower changes:

• the ovule becomes the seed (its outer layers harden into the seed coat);
• the zygote becomes the embryo; the endosperm stores food;
• the ovary becomes the fruit, and petals, stamens and often sepals wither and fall.

So, botanically, any structure that develops from an ovary and holds seeds is a fruit: tomatoes, cucumbers, pea pods, maple keys and sunflower "seeds" (each one a tiny dry fruit with one seed inside).

Seed dispersal moves seeds away from the parent so they don't compete with it for light and water:

• Wind: maple keys (samaras) spin; dandelion and milkweed seeds have fluffy parachutes.
• Animals: birds eat berries and drop the seeds elsewhere; burdock burrs hook onto fur; squirrels bury acorns and forget some.
• Water: some seeds float.
• Mechanical: ripe jewelweed (touch-me-not) pods burst when touched and fling seeds away.