Algae are the most primitive form of plant life. Most algae are aquatic, and they range in size from microscopic single-celled organisms to seaweed that is several metres long. Algae photosynthesize, like other plants, and they are responsible for providing most of the world’s oxygen. Algae are very varied, but even the large forms, such as kelp and other seaweeds, lack the true leaves, stems and roots found in other plants. Not all algae use the green chlorophyll found in other plants in order to photosynthesize; some use red or brown pigments for this purpose.
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