The flower and the structure of the anther

A flower is a seed factory

Look at a flower on a plant near you. The bright petals and the scent are not for us. They are for the insects and birds that visit it. The flower is the organ of sexual reproduction in a flowering plant, which botanists call an angiosperm. Every seed of a bean, a mango or a grain of rice began inside a flower.

The whole story of this chapter runs in three stages.

• Before fertilisation: the flower forms, the male and the female gametes are made, and pollen reaches the stigma. This is pollination.
• Fertilisation: a male gamete joins an egg.
• After fertilisation: the zygote grows into an embryo, the ovule becomes a seed, and the ovary becomes a fruit.

In this course we stay on the male side. We start by recalling the flower from Class 11 and then open the anther.

The parts of a flower

A flower sits on the swollen tip of its stalk, called the thalamus (also called the receptacle). On the thalamus there are four whorls. A whorl is a ring of parts that arise at the same level. From the outside to the inside they are the following.

• Calyx: the sepals, usually green.
• Corolla: the petals, often coloured.
• Androecium: the stamens, the male part.
• Gynoecium: one or more pistils, the female part.

A flower with both stamens and a pistil is bisexual. A flower with only one of them is unisexual. We now look at the androecium. The pistil comes in the next course.

The stamen and the anther

A stamen has two parts. The long, thin filament is the stalk, and its free end carries the anther. The anther is the part that makes pollen. The number and the length of the stamens differ from one species to another.

A typical anther is bilobed, which means that it has two lobes. The lobes are joined by a band of sterile tissue called the connective, which has a vascular bundle. Each lobe holds two microsporangia, so a typical anther has four of them. Because the anther has two thecae, one in each lobe, we say that it is dithecous.

A note on words. NCERT writes that each lobe has two theca. Most botany books say that the anther has two thecae, one in each lobe, and that each theca holds two microsporangia. The count to remember is the same in both: two lobes and four microsporangia.

In a cross section, the anther looks four-sided, which is called tetragonal. The four microsporangia lie at the four corners. As they grow, they become pollen sacs that run along the whole length of the anther and are packed with pollen grains.

The wall of a microsporangium

Now cut across one microsporangium. It looks like a circle with rings, as in a tree trunk.

The sac is surrounded by four wall layers. From the outside to the inside they are as follows.

The three outer layers protect the sac. They also help the anther to dehisce, which means to split open, so that the pollen is released. The tapetum is the innermost wall layer. Its cells have dense cytoplasm and usually more than one nucleus, because they are very active. They pass food to the cells that become pollen.

A way to remember the order from the outside: Every Elephant Must Tiptoe.

In the centre of a young anther there is a group of compactly arranged cells that look alike. This is the sporogenous tissue. Each of its cells is a potential pollen mother cell. The next lesson follows what happens to them.

You should find that the pollen of a flower grows in step with the number of stamens. Doubling the pollen mother cells in each sac doubles the grains in every sac.

Try it in Python

The program counts the pollen sacs and the pollen grains of one flower.

Practise with new numbers

Common mistakes

• Saying that an anther has two microsporangia. A typical anther has four, two in each lobe.
• Putting the tapetum on the outside. It is the innermost layer of the wall.
• Saying that the wall has three layers. There are four: epidermis, endothecium, middle layers and tapetum.
• Confusing the sporogenous tissue with the wall. The sporogenous tissue is the centre, and it makes the pollen.
• Mixing up the male and the female whorl. The androecium, made of stamens, is the male whorl.

Remember these

• The flower is the organ of sexual reproduction. The male whorl is the androecium, made of stamens.
• A stamen has a filament and an anther. A typical anther is bilobed, dithecous and four-sided, with four microsporangia.
• The wall of a microsporangium has four layers: epidermis, endothecium, middle layers and tapetum.
• The three outer layers protect and help the anther to dehisce. The tapetum feeds the pollen grains.
• The sporogenous tissue lies in the centre. Its cells can become pollen mother cells.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name the four whorls of a flower, from the outside to the inside.

2. How many microsporangia does a typical anther have, and where are they in a cross section?

3. List the four layers of the wall of a microsporangium from the outside to the inside.

4. Which layer feeds the developing pollen grains? What does it look like under the microscope?

5. What is the sporogenous tissue, and where is it?

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 1, Sexual Reproduction in Flowering Plants.
• Clark M. A. et al. (2018). Biology 2e, OpenStax, chapter 32, Plant Reproduction.
• Urry L. A. et al. (2017). Campbell Biology, 11th edition, Pearson, chapter 38.