External Structure and Importance of a Flowering Plant
Senior 1 · Biology
External Structure and Importance of a Flowering Plant
Lesson 1 of 5
25 min
On the tea estates of Nyamasheke and Rubavu, pickers move along the rows taking only "two leaves and a bud" from the top of each shoot. A week or two later the same bushes are covered in fresh shoots again, ready for the next picking. Where do the new shoots come from, when the tip has just been removed?
The answer lies in the structure of the shoot: stems carry buds at regular points, and each bud can grow into a new shoot. This lesson looks at that structure, starting with the whole plant.
Carefully dig up a young bean plant, keeping the roots whole, and wash off the soil. Lay it on a sheet of paper and find each part labelled in Figure 1. Use a hand lens to look for the small buds where the leaves join the stem, and measure the distance between two leaves with a ruler.
Figure 1: The external structure of a flowering plant (a bean plant).
Definition
Shoot system: all the parts of a plant above the ground: stem, leaves, buds, flowers and fruits. Root system: all the roots, usually below the ground.
Node: the point on a stem where a leaf is attached. Internode: the length of stem between two neighbouring nodes.
Bud: an undeveloped shoot. A terminal bud is at the tip of a stem; an axillary bud is in the axil, the upper angle between a leaf and the stem.
The stem holds the leaves up to the light and the flowers up where insects can reach them. Inside it, tubes carry water and mineral salts up from the roots and carry food (sugars) made in the leaves to every other part. Stems can be soft and green (herbaceous, like a bean) or hard and woody (like a eucalyptus trunk).
Every stem has nodes and internodes, and buds at the nodes. Roots never have nodes, leaves or buds, which is how you tell a stem from a root even when it grows underground.
| Part of the shoot | Main function |
|---|---|
| Stem | Supports leaves and flowers; carries water, minerals and food; green stems also photosynthesise |
| Terminal bud | Makes the stem grow longer |
| Axillary bud | Grows into a side branch or a flower |
| Leaf | Makes food by photosynthesis; exchanges gases; loses water vapour (transpiration) |
| Flower | Sexual reproduction: makes seeds |
| Fruit | Protects the seeds and helps to spread them |
A leaf is a thin, flat food factory. Its large, flat lamina catches sunlight; its veins bring water in and carry food out, and also stiffen the blade like the ribs of an umbrella.
Figure 2: The parts of a simple leaf.
The pattern of veins is called venation. There are two main types, shown in Figure 3.
Figure 3: Leaf venation types. Parallel veins are typical of monocots; net veins of dicots.
Definition
Lamina: the flat, broad part of a leaf (the leaf blade). Petiole: the stalk joining the lamina to the stem. A leaf without a petiole is sessile.
Midrib: the main vein running from the base to the apex. Venation: the arrangement of veins in a leaf, either parallel or net (reticulate).
Simple leaf: a leaf with one undivided lamina (coffee, avocado). Compound leaf: a leaf whose lamina is divided into separate leaflets on one petiole (bean, with three leaflets).
Warning
Leaflet or leaf? A common mistake is to count a bean's three leaflets as three leaves. Look for the axillary bud: a bud grows only in the axil of a whole leaf, where the petiole meets the stem, never at the base of a leaflet.
Worked example 1: simple or compound?
A student holds a bean shoot with three leaf-like blades on one stalk. Is it one leaf or three?
Answer: it is one compound leaf with three leaflets (a trifoliate leaf).
Worked example 2: internodes in the dark
Keza grows bean seedlings for two weeks, some in the light and some in a dark cupboard, then measures three internodes on each (in cm). Light: 2.1, 2.5, 2.3. Dark: 5.8, 6.4, 6.1. Compare the mean internode lengths and explain the difference.
Answer: internodes in the dark are almost three times as long (6.1 cm against 2.3 cm). A seedling without light puts its food store into growing tall and thin, which in nature would carry its leaves up to the light. Such seedlings are pale and weak: they are etiolated.
Now the tea puzzle can be solved. Plucking "two leaves and a bud" removes the terminal bud. With the tip gone, the axillary buds lower down on the shoot start to grow, and each one becomes a new shoot with young leaves to pick. Coffee farmers use the same idea when they prune: cutting back a stem makes the buds below the cut grow into new branches that will carry flowers and cherries.
Plants: Diversity, Structure, & Adaptations (Amoeba Sisters). Focus on the section about stems and leaves, and note the jobs they give each organ.
Tip
Memory hook: "Nodes Name stems." If a plant part has nodes, buds or leaves on it, it is a stem, even if it is underground (you will use this in lesson 5).
Practice: stems and leaves
Key points
Sources and credits
Figures 1–3, the table, the worked examples and the practice questions are written by Ganzaa.
