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Part VII : Reproduction in Flowering Plants

Flowers and Reproduction Summary (useful for revision)
Flowers and Reproduction : Questions
Laboratory work relating to this chapter

Flowers and Reproduction
The Structure of Insect Pollinated Flowers
Pollinating Mechanisms
Fruits and Seeds
Other Methods of Dispersal
Seed Germination
Conditions necessary for Germination
Development of the Embryo

Topic Chapters Index

 

Rye grass showing dangling anthers, Aust, UK © Shirley Burchill 

Rye grass showing dangling anthers, Aust, UK

 

Male flowers on maize plants © Shirley Burchill

Female flowers on maize plants © Shirley Burchill

Maize produces unisexual flowers

REPRODUCTION IN PLANTS

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Wind-pollinated Flowers

These flowers are often small and inconspicuous since they do not need to attract insects. They do not produce nectar and they do not have any scent. Most trees and grasses have wind-pollinated flowers.

 

Fact File No.34

Of the 250000 species of flowering plants only 10% (25000) are wind-pollinated. This 10%, however, includes all the cereal plants such as wheat, rice and maize. It also includes most of the trees found in forests.

One anther on a maize flower produces 3000 pollen grains. This means that all the anthers on one plant produce a total of around 18 million pollen grains!

 

Rye-grass flowers are small and very close together at the end of a long stem. In the summertime, if you look carefully enough, you can see the long, thin, anthers dangling out of the flowers. The stamens are loosely joined to the filaments and they vibrate even in the slightest breeze. They release large quantities of very small and light pollen grains which are easily carried away by the wind.

Other rye-grass flowers have mature stigmas which look like fine cotton wool as they dangle outside the flower. Some of the pollen is blown onto these stigmas where the pollen grains get caught in the network of threads.

 

The Pollen of Wind-pollinated Flowers

Wind-pollinated flowers need to produce great quantities of pollen to be sure that at least some pollen grains will reach the stigmas of other flowers. In the summertime many people suffer from `hay lever" which is like having a very bad cold. These people are allergic to the pollen in the air and sneeze a lot when they are outside. Hay fever can be treated with antihistamines which make the person suffering from hay fever less sensitive to the pollen. Newspapers and news broadcasters often give the pollen count which tells us how much pollen was in the air the previous day. On rainy days the pollen count is very low because rain washes the air clean of dust particles and pollen grains. On sunny, dry days the pollen count is high.

 

Unisexual Flowers

Some trees produce separate male and female flowers. Having unisexual flowers means that self-pollination is impossible. The hazel produces all its male flowers in a group to give a yellow structure which dangles down from the stem. These structures are often called catkins or 'lamb's tails'. If you lightly tap a catkin you will see clouds of pollen float into the air.

The female flowers are small and red with sticky, feathery-looking stigmas. As the pollen falls through the air some of the pollen grains will become trapped in the feathery stigmas.

 

Development of the Pollen Grain

Drawing of a carpel showing the pollen grains developing pollen tubes which are growing towards the ovules © Shirley Burchill

 

Pollination is only a stage towards fertilization (the meeting of the male and female sex cells). The male sex cell is inside the pollen grain which is on the surface of the stigma. The female sex cell (the ovule) is inside the carpel in a different part of the flower. The problem is how to get the male and the female sex cells together.

The pollen grains on the stigma start to produce a tube which grows into the stigma. The male sex cell is kept at the tip of this tube. The pollen tube grows through the stigma, style and into the carpel. Eventually it reaches an ovule and the male and female sex cells meet and fuse together. This is fertilization and the fertilized ovule now begins its development into a seed. When all the ovules have been fertilized the rest of the flower also changes. There is no longer any need for petals and these fall off. The anthers and sometimes the stigma and style also fall from the flower. The carpels start to grow larger as the seeds inside them develop. The whole structure is now called a fruit.

 

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