Plant classification and unicellular algae

Algae are the simplest (lower) plants: their body is not divided into root, stem and leaf and is called a thallus. They live mainly in water, have chlorophyll in their cells and make organic matter in light; besides chlorophyll they have other pigments, so blue-green, green, brown, red and yellow algae are distinguished. Tens of thousands of species are known; some are microscopic single cells, others several metres long. When unicellular algae gather, pond and ditch water turns green in summer (“blooming water”). Chlorella is a unicellular alga with a thin wall, cytoplasm, a nucleus and a green chromatophore; the chromatophore does the job of a chloroplast: in light it makes organic matter from water and CO₂ and releases oxygen into the water. Chlorella reproduces by cell division: 4 or 8 parts form inside the mother cell and each becomes a new Chlorella; it multiplies very fast and in autumn turns into a thick-walled spore to overwinter. Chlamydomonas swims with flagella, has a light-sensing “eyespot” and occurs in nitrogen-rich water.