DIATOMS, a sub-order of microscopic plants belonging to the class Alga. A plant of this sub-order consists of a unilocular or a septate cell; and its cells are composed of two symmetrical valves, multiplying by spontaneous separation. There are two well-defined sec tions: (1) Diatomece, including species invested with a silicious epidermal covering occurring both in fresh and salt water, often exhibiting exquisite sculpturings . when seen under the miscroscope and testing the highest powers of the instrument; (2) Desmidiece, minute fresh water plants of a green color, without a silicious covering. There are more than 4,000 distinct species scattered over all parts of the world. The walls of the cells are rendered hard by silica and they are reproduced by fission, the splitting taking place parallel to the longer axis. When the protoplasm escapes the cell walls retain their shapes and delicate markings. Enormous quantities of them are found as fos sils in the beds of the Tertiary formation. Often the protoplasm of two diatoms will escape and unite to form a zygospore, which after a time will divide into two, each forming a new cell larger than before. Some diatoms are free and move rapidly through the water; others secrete a mucilaginous substance whereby they attach themselves to various objects. In some cases the diatoms, after splitting, remain attached to one another, forming bands or ribbons. They are classified, according to the structure of the cell walls, three main families being recognized, according as they have on the principal face of the wall a distinct ridge (Raphidieee), an indis tinct or false ridge (Pseudoraphidieee), or no ridge at all (Araphidiece or Cryptoraphidiece.
The diatoms possessing a silicious epidermis have been eagerly studied of late years by mi croscopical observers, who have greatly in creased the number of genera and species. They are universally distributed and their silicious coverings being indestructible, their remains are accumulated and perpetuated in many localities, sometimes forming extensive as of Richmond, Va., which is said to be built upon a stratum of these miscroscopically minute atoms 18 feet in thickness. Species of Arachnoidiscus and other genera of great beauty are obtained from guano. The berg mehl or mountain-meal of Sweden consists of diatoms. The mud at the mouths of many rivers, the sediment of ponds, ditches and even rain-troughs, contain myriads of the same minute organisms. They were found by the Arc tic navigators investing the fields of polar ice; and they have also been detected in the dust evolved from volcanoes.
Their remains are found in rocks of many geologic ages, but are especially abundant in the Miocene beds of California. These organ isms are believed to have yielded the organic matter necessary for the formation of the rich petroleum deposits of that state. Diatoms are the source of the diatomaceous earth used in scouring soaps and polishing powders. Dia tomaceous oozes are now accumulating on the ocean bottom. See Ooze.