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Algæ (ăl′ jē).  One of the great divisions of Thallophytes (the lowest group of plants), being distinguished from the Fungi by containing the green coloring matter known as chlorophyll.  This enables them to manufacture their own food and so to live independently of all other organisms.  They are of special interest as representing the most primitive forms of the plant kingdom, from which all other groups of plants have probably been derived.  They are exclusively water plants, either living in the water or in damp places, and are commonly known as “seaweeds,” although they are abundant in fresh as well as in salt water.  Their bodies are of various sizes and degrees of complexity.  Some are only a single cell and are microscopic in size, while others are very complex and huge in size, as the giant kelps of the ocean.  There are four great groups of Algæ, named for their differences in color.  The Cyanophyæ or blue-green algæ are the simplest, and are characterized by possessing a blue pigment in addition to the green chlorophyll, which gives them a bluish-green hue.  The Chlorophyceæ or “green algæ” have no other pigment than the green chlorophyll.  These two groups are characteristic of fresh waters, although they have their marine representatives.  The two following groups are characteristic of salt waters, but have representatives in fresh waters.  The Phæophyceæ or brown algæ have a yellowish to brown pigment in addition to the chlorophyll, which gives their bodies various shades from olive to yellow and brown.  They include the common large and coarser seaweeds cast up by the waves.  The Rhodophyceæ or red algæ have a red pigment in addition to the chlorophyll, and their graceful and often very delicate bodies, beautifully tinted with various shades of red, are among the most attractive plants of the seashore.  For a further account see under the names of the four groups.