Flower structure: stamen, anther, pistil and ovule
The flower is the reproductive organ of angiosperms. The stamen (anther on a filament) makes pollen; the pistil (stigma, style, ovary) holds ovules that become seeds.
The flower and its reproductive whorls
In angiosperms (flowering plants), the flower is the site of sexual reproduction. Hormonal and structural changes turn a shoot tip into a floral primordium, which grows into a flower bud. Of the four whorls of a flower, two are reproductive. The androecium is the whorl of stamens, the male organs. The gynoecium is made of one or more pistils, the female organs. Large flowers such as hibiscus (china rose) and lily show these parts clearly.
Stamen and anther: parts and roles
| Part | What it is | Role |
|---|---|---|
| Filament | Long, slender stalk | Attaches the anther to the thalamus or a petal |
| Anther | Bilobed; each lobe has two thecae (dithecous); four-sided, with a microsporangium at each corner | The four microsporangia become pollen sacs |
| Epidermis, endothecium, middle layers | Outer three layers of the microsporangium wall | Protection, and help the anther split open (dehisce) to release pollen |
| Tapetum | Innermost wall layer | Nourishes the developing pollen grains |
| Sporogenous tissue | Compact cells at the centre of a young microsporangium | Each cell can become a pollen mother cell |
The pistil (gynoecium)
- The gynoecium may have one pistil (monocarpellary) or many pistils (multicarpellary).
- Pistils may be fused together (syncarpous, as in Papaver) or free (apocarpous, as in Michelia).
- Each pistil has a stigma (the landing platform for pollen), a style (the long, slender part) and an ovary (the swollen base).
- Inside the ovary is the ovarian cavity, or locule. The placenta inside it bears the ovules, also called megasporangia.
- The number of ovules varies: one in wheat, paddy and mango; many in papaya, watermelon and orchids.
Parts of an ovule (megasporangium)
| Part | What it is |
|---|---|
| Funicle | The stalk that attaches the ovule to the placenta |
| Hilum | The point where the body of the ovule joins the funicle |
| Integuments | One or two protective layers around the ovule |
| Micropyle | A small opening at the tip, where the integuments are absent |
| Chalaza | The basal part of the ovule, opposite the micropyle |
| Nucellus | A mass of cells inside the integuments, rich in reserve food |
| Embryo sac | The female gametophyte, inside the nucellus; usually one per ovule |
How to draw a T.S. of a young anther
- 1Draw a four-lobed outline like a butterfly, with two lobes on each side of a central connective.
- 2In each of the four corners, draw a round sac and label it microsporangium (pollen sac).
- 3Around one sac, draw four thin layers. Label them from outside: epidermis, endothecium, middle layers, tapetum.
- 4Fill the centre of each sac with small cells and label them sporogenous tissue.
- 5★Draw a small vascular bundle in the connective. Title: Transverse section of a young anther.
★ key step
How to draw an anatropous ovule
- Draw an oval body bent over so that its tip points down, lying beside its stalk.
- Draw the stalk and label it funicle; label hilum where it joins the body.
- Draw two layers around the oval and label them outer integument and inner integument. Leave a gap at the bottom and label it micropyle.
- At the base, opposite the micropyle, label chalaza.
- Shade the inside and label it nucellus. Draw a small embryo sac in it, near the micropylar end.
- Title: Diagrammatic view of a typical anatropous ovule.
Cut open a papaya and count its black seeds: each one was an ovule in the ovary. A mango has a single seed because its ovary holds only one ovule.
2 marks: “Name the wall layers of a microsporangium from outside to inside. What is the role of the innermost layer?” Epidermis, endothecium, middle layers, tapetum. The tapetum nourishes the developing pollen grains; the outer three layers protect and help dehiscence.
3 marks: “Draw a labelled diagram of a typical anatropous ovule.” Bent ovule with funicle, hilum, outer and inner integuments, micropyle, chalaza, nucellus and embryo sac; neat labels without crossing lines.
1 marks: “Why is a typical angiosperm anther called dithecous?” Each of its two lobes has two thecae.
Anther wall from outside: Every Eager Mind Thinks: Epidermis, Endothecium, Middle layers, Tapetum.