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Algae: the Simplest Green Plants

The main difference between green plants and other living things is their ability to make food - complex starch and sugar molecules from simple carbon dioxide and water. They do this by the process of photosynthesis which requires the presence of chlorophyll, the material that produces the colour in all green plants.

Algae are the simplest organisms in the plant kingdom that possess chlorophyll and can make their own food. Algae vary in size: they may consist of a single cell, or a row of cells joined together to form a filament. The largest algae consist of a plant body called a thallus, which is made of many cells and gives the plant a ribbon-like appearance. Algae live in both fresh and salt water and may swim, float or fasten themselves to a firm surface. Some of them live in the soil and a few are parasites on animals and plants. All algae possess chlorophyll, but their green colour may be masked by other pigments. This provides a way of classifying them according to colour, for instance green algae (Chlorophyceae)_, brown algae (Phaeo-phyceae) and red algae (Rhodophyceae). Algae reproduce in various ways. Sexual reproduction may occur, resulting in the formation of a new individual by the fusion of two sex cells. Sometimes asexual reproduction takes place, in which spores are produced, or a single-celled alga splits in two. Vegetative reproduction occurs when a filament or thallus breaks into two parts to form new plant bodies.

One of the most common of the single-celled green algae is Pleurocoocus, whose colonies form a green film over the shady, moist side of tree trunks. Spirogyra is a common filamentous green alga which grows in ponds and stagnant pools. A salt-water alga with a thallus body is the sea lettuce, Ulva, which is eaten in salads in some parts of the world. Diatoms are single-celled algae (Bacillariophyceae), that live in fresh in salt water. They grow in enormous numbers and make up a great part of the floating organisms called plankton, which is found in all seas and is the food supply for many aquatic animals.

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