External Structure and Importance of a Flowering Plant
Senior 1 · Biology
External Structure and Importance of a Flowering Plant
Lesson 3 of 5
25 min
In the rainy season, the Nyabarongo river runs brown: it is carrying soil washed off bare fields upstream. Yet on terraced hillsides where farmers have planted strips of grass along every terrace edge, the soil stays in place even in heavy rain. Above ground, a grass strip is only a few leaves. The difference is below ground, in the roots.
Part 1. Dig up a bean seedling and a maize seedling, each about four weeks old, with a trowel, keeping a large ball of soil around the roots. Soak each ball in a bucket of water until the soil falls away, then lay the roots on newspaper and compare them.
Part 2. Germinate a few bean seeds on damp blotting paper in a covered dish. After four days, look at a young root with a hand lens against a dark background. You will see a white "fuzz" a little way behind the tip, but none at the tip itself.
Figure 1: The two types of root system, drawn from seedlings about four weeks old.
Results. The bean has one thick main root growing straight down, with thinner roots branching from it. The maize has no main root: dozens of roots of about the same thickness grow from the base of the stem, forming a dense mat in the top layer of soil. The white fuzz on the germinating bean is made of root hairs.
Definition
Root: the part of a plant, usually underground, that anchors it and absorbs water and mineral salts. Roots have no nodes, buds or leaves.
Tap root system: one main (tap) root that grows straight down, with smaller lateral roots branching from it. Typical of dicots (bean, avocado, eucalyptus).
Fibrous root system: many thin roots of similar size growing from the base of the stem, with no main root. Typical of monocots (maize, sorghum, grasses).
Adventitious root: a root that grows from a stem or leaf instead of from another root, for example maize prop roots.
Root hair: a long, thin outgrowth of a single surface cell of the root, which absorbs water and minerals. Root cap: a cap of cells that protects the root tip as it pushes through soil.
Figure 2: A root tip, magnified. Root hairs grow only in the older zone behind the tip; the tip itself is protected by the root cap as it pushes through the soil.
Root hairs matter because there are so many of them. Thousands of hairs on each root greatly increase the surface area in contact with the soil water, so far more water and minerals can be absorbed than by the root's smooth surface alone. Each hair is a single cell and lives only a few days; new ones form continuously just behind the growing tip.
| Feature | Root | Stem |
|---|---|---|
| Nodes, buds and leaves | absent | present |
| Tip protected by | a root cap | a terminal bud |
| Usual direction of growth | down, towards gravity and water | up, towards light |
| Colour | not green (no chlorophyll) | often green when young |
| Hairs for absorption | root hairs | none |
Warning
Root hairs are not small roots. A lateral root is a whole organ with many cells, a root cap and its own root hairs, and it lasts as long as the plant. A root hair is one single cell, too thin to see without a hand lens, and it lives only a few days.
Worked example 1: which root system?
Predict the root system of (a) sorghum, whose leaves have parallel veins, and (b) an avocado tree, whose leaves have net veins.
Answer: (a) sorghum: fibrous root system; (b) avocado: tap root system. Features of one organ often predict features of another, because they belong to the same plant group.
Worked example 2: why transplanted seedlings wilt
Mutesi pulls cabbage seedlings out of a nursery bed with bare roots and plants them in her field at midday. By evening they have wilted. Explain why, and suggest how she should transplant next time.
Answer: lift the seedlings with a ball of soil so the root hairs stay in contact with soil, transplant in the evening or on a cloudy day, and water them well straight away.
This is why grass strips protect terraces. The fibrous roots of grasses such as Napier grass form a dense net in the top 30 cm of soil and bind it together, so rain cannot carry it away; the grass is also cut as cattle fodder. Trees with deep tap roots, such as Grevillea planted along the edges of fields, anchor deeper layers and reach water far below the surface. Rwanda's programmes of terracing and tree planting use both kinds of root system.
Root System (7activestudio). Watch for the difference between tap roots and fibrous roots, and where the root hairs grow.
Tip
Memory hook: a tap root is like a tap in a wall: one pipe that goes deep. A fibrous root system is like a fibre mat, or a head of hair: many thin strands spreading near the surface.
Practice: the root system
Key points
Sources and credits
Figures 1 and 2, the investigation, the table, the worked examples and the practice questions are written by Ganzaa.
