Flowers are present in a great variety of sizes, shapes, and colours. They can develop as single flowers or in groups called inflorescences.
Flowers are part of the plant shoot system (including non-reproductive leaves, stems, and reproductive flowers) and develop from a stalk called a peduncle.
In inflorescences, each flower has a pedicel that attaches them to the main axis. The tip of a peduncle or a pedicel is the receptacle, which contains the flower parts.
Tulips, roses, and magnolias are relatively big individual flowers. Snapdragons and lilac are inflorescences.
What about sunflowers? They look like big individual flowers; however, sunflowers are a type of inflorescence! Sunflowers, daisies, dandelions, and many others belong to the Asteraceae family, in which the floral receptacle contains numerous tiny flowers.
The flowers at the border that look like yellow petals are small incomplete, and sterile flowers (they do not have stamens or pistils). These surround hundreds of smaller flowers in the centre containing reproductive components. Besides, the tiny flowers develop on a big receptacle and do not have individual pedicels.
Fig. 2 - Flower diversity. From left to right: a rose develops as an individual flower; the next three images show different types of inflorescences: lilac, sunflower, and grass.
Plants have evolved diverse mechanisms to accomplish that the sperm get to the egg, and in most flowering plants, the flower is central to this task. Plants make use of pollinating agents to move the pollen among flowers. While some plants use wind or water for pollination, the great majority rely on animals (insects, birds, and mammals) to transport the pollen.
Pollination is the transfer of pollen from the male reproductive structure to the female reproductive structure in seed plants.
Angiosperms use flowers to attract pollinators. Most of a flower’s traits (size, colour, nectar, scent) are for attracting a type of pollinator (or not attracting it, if it uses wind for pollination), sometimes even a specific animal species.
Colourful flowers typically attract birds and some insects. But some plants have inconspicuous flowers. Have you seen grass flowers? They pass unnoticed by most people. Grass belongs to the family Poaceae, a big, diverse group that includes grass, pastures, cereal grasses, and bamboo. The wind or insects pollinate most grasses and thus, have typically very small, colourless, or beige-ish flowers (see figure 1, the last image at the right is from a grass). You will learn more about Pollination here.
What are the steps after pollination?
Irrespective of the pollinating agent, pollen grains are transferred from a flower anther to the stigma of another flower. Once on the stigma, the pollen grain germinates, developing a pollen tube. The pollen tube is a long and slender tube that goes all the way down through the style and reaches the interior of the ovary. The sperm (which is inside the pollen grain) moves through this tube to the ovary (Figure 3).
Figure 3. Pollen germination (1) and fertilization (2). Source: modified from LadyofHats, Public domain, via Wikimedia Commons.
The ovary contains one or more ovules. Each ovule contains an embryo sac with a few cells including one egg cell. One sperm fertilizes the egg cell forming a zygote. The zygote develops into the plant embryo, contained in the ovule that develops into the seed.
Fertilization in angiosperms is called double fertilization.
The seed or seeds are enclosed by the ovary that grows to become the fruit. The fruit helps in seed dispersal, and its characteristics (colourful and nutritious or inconspicuous and weightless) depend on the dispersal method. When the seed germinates, a new plant develops, and the cycle starts again.
Fruits can be colourful and nutritious (and delicious!) like apples, mangoes, guavas, and figs, to attract animals that eat them and disperse the seeds. On the other hand, some fruits are also dispersed by animals not by eating them but by transporting them in their fur or feathers. In this case, fruits have to be small and have shape modifications or structures that allow them to stick to an animal's fur or feathers when they walk by (like burs that have spikes or hooks). Likewise, seeds dispersed by wind are small, weightless, and modified to be able to “fly” with the wind (like a dandelion seed with its feathery fruit).
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