Unit 3 External structure and importance of a flowering plant
Key unit competence: Differentiate flowering plants based on their external structures.
Key ideas
- A flowering plant has a shoot system above the ground (stem, leaves, buds, flowers, fruits) and a root system below the ground. Flowering plants all produce flowers, fruits and seeds.
- Monocotyledons (maize, sorghum, rice, grasses, banana) have one seed leaf, fibrous roots, narrow leaves with parallel veins and flower parts in threes. Dicotyledons (bean, mango, avocado, black jack, hibiscus) have two seed leaves, a tap root, broad leaves with a network of veins (reticulate venation) and flower parts in fours or fives.
- A leaf has a stalk (petiole), a flat blade (lamina), a main vein (midrib) and veins. The thin, broad lamina gives a large surface area to trap light and take in carbon dioxide; veins bring water and carry away sugar; the waxy cuticle reduces water loss; stomata allow gas exchange.
- Leaf arrangement on the stem (phyllotaxis): alternate (one leaf per node), opposite (two leaves at a node) and whorled (three or more at a node). Arrangement helps leaves avoid shading each other.
- Parts of a stem: node (where leaves and buds grow), internode (part between two nodes), apical (terminal) bud (growth in length), axillary bud (makes branches or flowers), lenticels (pores for gas exchange). The stem supports leaves and flowers, transports water and food, may store food and, if green, photosynthesises.
- The flower is the reproductive organ: sepals protect the bud; petals attract pollinators; stamens (anther and filament) make pollen; the carpel/pistil (stigma, style, ovary) receives pollen and holds the ovules. After fertilisation, ovules become seeds and the ovary becomes the fruit.
- Roots: a tap root system has one main root with lateral roots (bean, carrot); a fibrous system has many similar thin roots from the stem base (maize, grasses). Parts: primary root, lateral roots, root hairs (large surface for absorbing water and minerals), root cap (protects the tip), growing point. Functions: anchorage, absorption, conduction, storage.
- Modified stems: stem tuber (Irish potato), rhizome (ginger), runners/stolons (strawberry, couch grass), suckers (banana), tendrils, thorns, green photosynthetic stems (cactus). A stem is recognised by nodes, internodes and buds ("eyes" of a potato).
- Modified leaves: spines (cactus) reduce water loss and protect; fleshy leaves store water (aloe, succulents); scale leaves store food (onion bulb); tendrils help climbing. Modified roots: storage roots (carrot, sweet potato, cassava), prop roots for support (maize, sugarcane), buttress roots (large trees), aerial roots (gas exchange).
- Importance of flowering plants: food (maize, beans, bananas, rice), oxygen and removal of carbon dioxide, timber and firewood, medicines (quinine, aloe), fibres (cotton, sisal), shelter for animals, beauty and income (floriculture, coffee and tea exports).
Common misconceptions
- "A potato is a root because it grows underground." No: a potato has nodes and buds (eyes), so it is a stem tuber. A carrot and a sweet potato are roots.
- "The fruit grows from the petals." No: the fruit develops from the ovary; the petals usually fall off.
- "Root hairs are small roots." No: a root hair is a long extension of a single outer cell of the root; it increases the surface for absorption.
- "Ginger is a root." No: ginger is a rhizome, an underground stem with nodes, internodes and scale leaves.
Worked examples
Example 1. A learner is given a plant with parallel-veined leaves and fibrous roots. Classify it and give two more features you would expect.
Solution
It is a monocotyledon. Its seed would have one cotyledon, and its flower parts would be in threes or multiples of three. Examples: maize, sorghum.
Example 2. Explain how a cactus survives in a dry place without true leaves.
Solution
Its leaves are reduced to spines, so little water is lost by transpiration, and spines protect it from animals. Its thick green stem photosynthesises and stores water, and a thick waxy cuticle reduces evaporation.
Practice questions
State four differences between monocotyledons and dicotyledons.
Show answer
Monocots: one cotyledon, fibrous roots, parallel veins, flower parts in threes. Dicots: two cotyledons, tap root, network of veins, flower parts in fours or fives.
Common mistake: Writing a feature for one group without its opposite for the other group.
Name the parts of a flower and give the function of each.
Show answer
Sepals protect the bud; petals attract pollinators; anther makes pollen, filament holds the anther; stigma catches pollen, style connects stigma to ovary, ovary holds ovules; receptacle holds the flower parts.
Common mistake: Mixing up stigma (catches pollen) and anther (makes pollen).
Explain how the structure of a leaf suits it for photosynthesis.
Show answer
Broad, thin lamina: large surface to trap light and short distance for gases; chlorophyll traps light; veins bring water and remove sugar; stomata let carbon dioxide in.
Common mistake: Listing parts without linking each one to photosynthesis.
State four functions of roots.
Show answer
Anchor the plant; absorb water and mineral salts; conduct water to the stem; store food (carrot, cassava). Some give extra support (prop roots).
Common mistake: Saying roots carry out photosynthesis.
You are given a potato, a carrot and ginger. Identify the stems and the root and give a reason.
Show answer
Potato and ginger are stems because they have nodes and buds. The carrot is a root: it has no nodes or buds and grows from the radicle.
Common mistake: Classifying by "grows underground" alone.
Give four ways in which flowering plants are important to people in Rwanda.
Show answer
Food (maize, beans, bananas, cassava); cash crops and exports (coffee, tea); timber and firewood; medicines; they produce oxygen and protect the soil from erosion.
Common mistake: Giving only food examples.
Explain how modified stems help farmers to produce new crops quickly.
Show answer
Stems such as potato tubers, sugarcane cuttings, banana suckers and strawberry runners have buds that grow into new plants (vegetative propagation). The new plants grow fast and are identical to good parent plants.
Common mistake: Saying modified stems produce seeds.
Distinguish between a tap root system and a fibrous root system, with one example each.
Show answer
Tap root: one main root grows down with smaller lateral roots (bean, carrot). Fibrous root: many thin roots of similar size grow from the stem base (maize, grass).
Common mistake: Giving maize as an example of a tap root.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Which part of a flower attracts insects? a. fruit 2. After fertilisation, the ovary becomes the: b. fleshy scale leaves 3. Which plant product is used to make medicine against malaria? c. Quinine 4. An onion bulb stores food in its: d. Petals Show answer
1 → d (Petals) · 2 → a (fruit) · 3 → c (Quinine) · 4 → b (fleshy scale leaves)
- Bright, scented petals attract pollinators.
- Ovules become seeds; the ovary becomes the fruit.
- Quinine comes from the bark of the cinchona tree.
- The bulb is made of swollen leaf bases.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
reduces water loss · petiole · stem tuber · makes food- An Irish potato is a .
- The stalk that attaches a leaf to the stem is the .
- The waxy layer on the surface of a leaf mainly .
Show answer
1: stem tuber · 2: petiole · 3: reduces water loss
- It has nodes and buds ("eyes").
- The petiole joins the lamina to the stem at a node.
- The cuticle is waterproof.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. The structures that greatly increase the surface area of a root for absorption are: a. node 2. The flat, broad part of a leaf is the: b. lamina 3. The point on a stem where a leaf grows is called a: c. Bean 4. Which plant is a dicotyledon? d. root hairs Show answer
1 → d (root hairs) · 2 → b (lamina) · 3 → a (node) · 4 → c (Bean)
- Many fine root hairs make a large surface for water and mineral uptake.
- The lamina gives a large surface area for photosynthesis.
- The internode is the part between two nodes.
- Beans have two cotyledons, a tap root and net-veined leaves.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
alternate arrangement · runners (stolons) · parallel arrangement · rhizome- Leaves arranged with one leaf at each node have .
- Ginger is an example of a .
- Strawberry plants spread by .
Show answer
1: alternate arrangement · 2: rhizome · 3: runners (stolons)
- Opposite has two leaves per node; whorled has three or more.
- A rhizome is a horizontal underground stem.
- Runners grow along the ground and form new plants at their nodes.