Many of the plants that we see around us grow from seeds that have been produced within the ovaries (seedboxes) of the flowers. More than half the world's plants bear flowers. They are known as flowering plants or angiosperms.
The two main groups of flowering plants or angiosperms are dicotyledons and monocotyledons, or dicots and monocots. The dicots which outnumber monocots in species by four to one, are called by that name because they have two(di) cotyledons -- tiny seed leaves that are a part of the embryo plant while it still exists within the seed. Monocots are so called because they have only a single cotyledon. Of all the plants on the earth, these monocots stand out as the ones that have adapted most completely to their own environment. Leaves of dicots vary greatly in shape and display a network of leaf veins which gives the leaf greater physical support. Monocot leaves have simple, elongated shapes and parallel veins. The dicot stem has a central pith ringed by water-carrying tubes while the monocot stem is simpler, often soft with random vascular bundles. Dicot flowers usually have petals in groups of fours and fives while monocot flowers have three petals or multiples of three.
Parts of a flower
A flower has four parts: sepals, petals, stamen and gynoecium. The sepals, usually green, together form the calyx which protects the flower bud before it opens. The petals are usually the brightly coloured leaf-like flower parts which together form the corolla. The stamen is the male part of a flower. It is made up of a slender stalk called the filament which has an enlarged tip called the anther. The female portion of a flower, gynoecium, has three parts. At the bottom is the ovary where there are one or more ovules. The style rises from the ovary. At the top of the style or its branches is a sticky area, the stigma.
How seeds are formed
In order for seeds to be produced, pollen from the anther must reach the stigma. Once the pollen grains have landed on the stigma of the same species of plant, they take in water, swell up and break open. From each grain, a tiny pollen tube develops, from instructions sent out by one of the nuclei in the pollen grain. This tube digests away the style as it grows toward the ovary. Along the way, the other nucleus in the pollen grain divides into two sperm nuclei.
When the pollen tube reaches the bottom of the style, it breaks through into the ovary. Each ovule holds an egg cell and a group of other cells. Upon reaching an ovule, the tip of the pollen tube splits open and releases the two sperm nuclei. One sperm nucleus unites with the egg cell, fertilising it. The fertilised egg develops into an embryo. The second sperm nucleus enters another cell with two nuclei in the ovule to form the endosperm. The endosperm is absent in some seeds. The ovule then becomes a seed.
