ALGAE. The algae comprise seaweeds, pond-scums, and a great variety of microscopic plants inhabiting fresh and salt waters in countless numbers. Many are found in or on the soil or on rocks and tree-trunks, and no region of the earth is devoid of them (see Occurrence and Distribution, p. 6oi ). The Latin alga means seaweed and only later acquired its present wider meaning. The algae belong to the lowest division (Thallophyta) of the vegetable kingdom, and are distinguished from the majority of higher plants by their simple reproductive organs and by the fact that their vegetative body as a general rule has a relatively simple construction; it is usual to speak of the body of the bigger algae as a thallus. All algae are holophytic; i.e., they contain chlorophyll which enables them to use the energy of sunlight and to build up in the process of photosynthesis the organic compounds forming their body from carbon dioxide, water, and other simple sub stances. This distinguishes them from the fungi, the other sub division of the Thallophyta. On the other hand it is very difficult to separate the algae from those groups of Flagellata which are holophytic (see Phylogeny, p. 600, and PROTOPHYTA) . Most algae can only be studied with the help of a microscope, although no very high powers are necessary to make out the essential features.