Diatoms

Diatoms, a numerous group of microscopic plants, sometimes classed among Algae (q.v.), but now ranked as an independent class of the Thallophyta (q.v.). They are unicellular, and have within their cell-wall a siliceous case composed of two halves or valves, one fitting over the other like the lid over a pill-box. They vary in size from less than one-third of a millimetre to three millimetres in length, the marine forms being the largest. In external form they are extremely varied, being round, oval, linear, fusiform, crescent-shaped, cuneate, or S-shaped (sigmoid); and their siliceous cases exhibit remarkably varied minute geometrical patterns as if of ornamentation, which have been used by microscopists to test the power of their lenses. Some diatoms occur on moist rocks, others in pools or streams on mud- or water-plants, and others in brackish or salt water on algae or in the stomachs of marine animals. Some are free, others loosely united in chains, and others attached by gelatinous threads to foreign bodies. Some species abound in the ice and deep waters of polar regions, whilst others occur in hot springs. Within their cells are plate-like chloroplastids; but the green colour is masked by a brown substance known as diatomin. Diatoms multiply rapidly by bipartition, each individual or frustule separating into two valves, each of which forms a new but slightly smaller half. After several such bipartitions, a larger cell or auxospore is formed, and then the process recommences. In some cases the auxospore results from a true process of conjugation, and is, therefore, a zygospore (q.v.). Vast "deposits composed almost exclusively of diatoms have been found in many places, most being of Tertiary age. That at Richmond, Virginia, is 40 feet thick, and extends for many miles. Under the names tripoli, polishing powder, mountain meal, kiesel-guhr, and dynamite-earth, they are extensively employed as a polishing material or as an absorbent for nitro-glycerine in the manufacture of dynamite (q.v.).