actin(o)- Also actini-.

A ray or beam.

[Greek aktis, aktin-, ray.]

Some examples refer to rays of light. Radiation that is able to cause chemical reactions, as light does on photographic film, is said to be actinic; an actinometer is an instrument for measuring the intensity of radiation. Actinium is a rare radioactive element (atomic number 89), so named because it glows in the dark; hence actinide, a member of the series of radioactive elements, including uranium and plutonium, of which actinium is the first.

Other examples refer to structures that radiate from a centre. Something actinomorphic, such as a starfish or the flower of a daisy, has radial symmetry. An actinomycete (Greek mukēs, mukēt-, fungus) is a bacterium with a filamentous form, which was formerly considered to be a fungus, the ray-fungus; these can cause disease in humans and animals; actinomycin is a member of a group of antibiotics extracted from soil bacteria of this genus.

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