Another clearly defined characteristic of plants is that they possess true roots, stems and leaves. Seaweed contains a simple arrangement of cells forming a 'holdfast', stipe (primitive stem) and various fronds. They photosynthesise effectively but, owing to their simple specialisation and undifferentiated cell structure, are not classified as plants. They are in fact algae and classified alongside other single-celled structures such as Amoeba, Euglena and Paramecium. Pupils often describe single-celled plant-like and animal-like organisms as being true plants or true animals. Lichens are a result of a symbiotic association between two groups of organism - fungi and algae - and are not classified as plants either. However, it is useful to see all photosynthetic organisms, whether single-celled or true plants, as 'producers' on which 'consumers' depend.
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