External Structure and Importance of a Flowering Plant

Lessons

Senior 1 · Biology

External Structure and Importance of a Flowering Plant

5 lessons

Lesson 5 of 5

Modified roots, stems and leaves, and why flowering plants matter

25 min

Is an Irish potato a root?

Farmers in Musanze and Nyabihu dig Irish potatoes (ibirayi) out of the ground; farmers in the south dig up cassava (imyumbati) and sweet potatoes. All three grow underground and are full of starch, so most people call them all "roots". A biologist disagrees: one of them is really a stem. Which one, and how can you prove it?

Investigation: roots or stems?

Take an Irish potato, a piece of cassava tuber and an onion. Look at each with a hand lens.

Look for eyes (small hollows with a bud) and any small scale-like leaves or leaf scars.
Leave the potato in a bright place for a week and watch where shoots appear, and whether the skin turns green.
Cut the onion in half from top to bottom and draw what you see.

The potato sprouts shoots from its eyes, and its skin turns green in the light. The cassava tuber has no eyes at all. The onion is made of layers wrapped around a small flat base.

Figure 1: A stem tuber and a root tuber. The potato's eyes are buds at nodes, which proves it is a stem; the cassava tuber has none.

Definition

Modified organ: a root, stem or leaf whose structure has changed to carry out a special job, such as storing food, climbing or protection.

Stem tuber: the swollen tip of an underground stem that stores food and has buds (eyes) at its nodes (Irish potato). Root tuber: a swollen root that stores food and has no nodes or buds (cassava, sweet potato).

Rhizome: a horizontal underground stem with nodes and scale leaves (ginger). Bulb: a very short stem surrounded by fleshy, food-storing leaf bases (onion).

Tendril: a thin, coiling stem or leaf part that wraps round supports so the plant can climb (passion fruit, garden pea).

Why roots, stems and leaves are modified

Many plants in Rwanda face a dry season. Storing food underground lets them survive it and grow again quickly when the rains come, and the stored food (mostly starch) is exactly what makes these organs valuable to people.

Figure 2: An onion bulb in section. A bulb is a very short stem surrounded by fleshy leaves that store food.

Modified organ	Organ it comes from	Example in Rwanda	Job
Stem tuber	stem	Irish potato	food store; new plants from the eyes
Rhizome	stem	ginger	food store; spreads underground
Corm	stem	taro (cocoyam)	food store
Stem tendril	stem	passion fruit	climbing
Thorn	stem	bougainvillea	protection; scrambling over other plants
Root tuber	root	cassava, sweet potato	food store
Prop root	root	maize	extra support
Bulb	leaf bases	onion	food store
Leaf tendril	leaf	garden pea	climbing
Succulent leaf	leaf	aloe	water store in dry places

Warning

Underground does not mean root. Decide by structure, not by position: anything with nodes, buds (eyes) or scale leaves is a stem, even if it grows in the soil (potato, ginger, the base of an onion). A swollen organ with none of these is a root (cassava, sweet potato).

Worked example 1: proving the potato is a stem

List the evidence that an Irish potato is a stem tuber, not a root.

Buds: each eye contains buds that grow into shoots. Roots have no buds.
Leaves: each eye sits above a tiny scale-leaf scar, so eyes are at nodes. Roots have no nodes.
Position: the tuber forms at the tip of an underground stem (stolon), not from a root.
Chlorophyll: in light, the skin turns green, as a stem can; this green skin contains a poison (solanine), so green potatoes should not be eaten.

Answer: four independent pieces of evidence show that the potato is a swollen stem.

Worked example 2: planting from tubers

Habimana has a seed potato with 6 eyes. He cuts it into pieces to plant. What is the most plants he can grow from it, and what must each piece contain?

A new shoot can only grow from a bud, and the buds are in the eyes.
Each piece must therefore contain at least one eye, plus some stored food to feed the shoot until its leaves can photosynthesise.
With 6 eyes, at most 6 pieces can each have an eye.

Answer: at most 6 plants, one from each piece with an eye. A piece of cassava tuber planted in the same way would not grow at all, because it has no buds; cassava is planted from stem cuttings instead.

Why flowering plants matter to Rwanda

Food: beans, maize, bananas, cassava, Irish potatoes, sweet potatoes and sorghum provide most of the food eaten in Rwanda.
Export earnings: coffee and tea are among the country's largest exports, and pyrethrum flowers from the Northern Province are made into a natural insecticide.
Fuel and building: eucalyptus and grevillea give firewood, charcoal, poles and timber; bamboo, a giant grass, is used for building and furniture.
Animal feed: Napier grass and other fodder plants feed the cows kept by many families.
Crafts: agaseke baskets are woven from sisal and sweetgrass; papyrus from the wetlands is made into mats.
Medicines: the malaria drug artemisinin comes from the plant Artemisia annua, and many modern medicines began as plant remedies.
Environment: plants make oxygen, store carbon, hold soil on the hills, and give homes and food to wildlife in Nyungwe and Akagera.

Modification of root, stem and leaf (Macmillan Education India). Watch for how each organ is identified as a root, stem or leaf, and what job each modification does.

Tip

Memory hook: "If it has eyes, it's a stem in disguise."

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Practice: modified organs and the importance of plants

Define a modified organ and give one example of a modified root, stem and leaf.
Give two pieces of evidence that an Irish potato is a stem.
Which part of an onion is the stem, and which parts store food?
Why is cassava grown from stem cuttings and not from pieces of its tuber?
State four ways in which flowering plants are important to Rwanda, with an example of each.
A student says ginger is a root because it grows underground. Evaluate this statement.

Show answersHide answers

An organ changed in structure to do a special job. Root: cassava tuber (or maize prop root). Stem: Irish potato (or ginger, passion-fruit tendril). Leaf: onion bulb leaves (or pea tendril, aloe).
Any two: it has eyes (buds) at nodes; it has scale-leaf scars; it forms at the tip of an underground stem; it turns green in light.
The short flat disc at the base is the stem; the fleshy leaf bases store food.
The tuber is a root with no buds, so it cannot grow a shoot; stem cuttings have nodes with buds.
Any four, e.g. food (beans, maize); exports (coffee, tea); fuel and timber (eucalyptus); fodder (Napier grass); crafts (sisal for agaseke); medicine (Artemisia); soil conservation (grasses on terraces).
Incorrect: position does not decide the organ. Ginger has nodes, scale leaves and buds, so it is a stem (a rhizome).

Key points

Roots, stems and leaves can be modified for storage, support, climbing or protection.
Stems (even underground ones) have nodes and buds; roots never do: the potato is a stem tuber, cassava a root tuber, the onion a bulb of leaves.
Flowering plants give Rwanda its food, exports, fuel, fodder, medicines and healthy soils.

1Is an Irish potato a root?

2Investigation: roots or stems?

3Why roots, stems and leaves are modified

4Why flowering plants matter to Rwanda

Is an Irish potato a root?

Investigation: roots or stems?

Why roots, stems and leaves are modified

Why flowering plants matter to Rwanda