Ferns, flowering plants, viruses and keys

Last time

Before we start: last time, in Characteristics of living organisms, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Three new tools

Today, three short ideas. How to tell ferns from flowering plants, and monocots from dicots. Why viruses are not in any kingdom. And how to use a dichotomous key to name an organism you have never seen.

Ferns

Ferns are plants with roots, stems and leaves. Their leaves are called fronds. Ferns do not make flowers or seeds. They reproduce by spores. You often see the spores as brown spots on the underside of a frond.

Flowering plants

Flowering plants make flowers and seeds. They are split into two groups by their seeds and leaves. Monocotyledons have one seed leaf, long narrow leaves with parallel veins, and flower parts in threes. Dicotyledons have two seed leaves, broad leaves with a branching network of veins, and flower parts in fours or fives.

Monocot or dicot?

Here is the comparison. Grass, maize and lilies are monocots. Beans, roses and sunflowers are dicots. Count the flower parts and look at the veins.

Viruses

A virus is not made of cells. It is a protein coat around genetic material, DNA or RNA. It has no cell membrane, no cytoplasm, no nucleus, no ribosomes and no mitochondria. It can only reproduce inside a living host cell, using the host's machinery. Because it shows none of the characteristics of life on its own, it is not placed in any kingdom.

How a virus makes more of itself

The virus attaches to a host cell and injects its genetic material. The host cell is forced to make new virus genes and new protein coats. New viruses are assembled, and the cell bursts to release them.

Dichotomous keys

A dichotomous key identifies an organism by a series of steps, each with two choices. Each choice is about a feature you can see, such as the number of legs or the shape of a leaf. You follow the choice that fits, step by step, until you reach a name.

Using a key

Here is a short key. Start at step one: does it have legs? An animal with legs goes to step two: more than three pairs of legs? Four pairs leads to spider. Three pairs leads to insect.

Making your own key

To build a key, choose a feature that splits the organisms into two groups. Write it as a pair of opposite statements: wings, or no wings. Keep splitting each group until every organism is on its own. Never use features like size or colour that change, or that need a comparison.