COHN, FERDINAND JULIUS (1828-1898), German botanist, was born on the 24th of January 1828 at Breslau. He was educated at Breslau and Berlin, and in 18J9 became extraordinary, and in 1871 ordinary, professor of botany at Breslau University. He was contemporary with N. Pringsheim, and worked with H. R. Goeppert, C. G. Nees von Esenbeck, C. G. Ehrenberg and Johannes Muller. He made remarkable advances in the estab lishment of an improved cell-theory, discovered the cilia in, and analysed the movements of, zoospores, and pointed out that the protoplasm of the plant-cell and the sarcode of the zoologists were one and the same physical vehicle of life. Although these early researches were especially on the Algae, in which group he insti tuted marked reforms of the rigid system due to F. T. KUtzing, Cohn studied such varied subjects as Aldorovanda, torsion in trees, the nature of waterspouts, the effects of lightning, physiol ogy of seeds, the proteid crystals in the potato, which he discov ered, formation of travertin, the rotatoria, and luminous worms.
It is, however, in the introduction of the strict biological and philosophical analysis of the life-histories of the lower and most minute forms of life that Cohn's greatest achievements consist, for he applied to these organisms the principle that we can only know the phases of growth of microscopic plants by watching every stage of development ' under the microscope, just as we learn how different are the youthful and adult appearances of an oak or a fern by direct observation. His account of the life histories of Protococcus (1850), Stepkanosphaera (1852), Volvox (1856 and 1875), Hydrodictyon (1861), and Spaeroplea 18S 7) among the Algae have never been put aside. The first is a model of what a study in development should be ; the last shares with G. Thuret's studies on Fucus and Pringsheim's on Vaucheria the merit of establishing the existence of a sexual process in Algae. Among the Fungi Cohn contributed important researches on Pilobolus (1851), Empusa (1855), Tarichium (1869), as well as valuable work on the nature of parasitism of Algae and Fungi.
Cohn may be said to have founded the science of bacteriology. He seems to have been always attracted particularly by curious problems of fermentation and coloration due to the most minute forms of life, as evinced by his papers on Monas prodigiosa (1850) and "Ober blutahnliche Farbungen" (185o), on infusoria (1851 and 1852), on organisms in drinking-water (1853), "Die Wunder des Blutes" (1854), and had already published several works on insect epidemics (I869-187o) and on plant diseases, when his first specially bacteriological memoir (Crenothrix) ap peared in the journal, Beitriige zur Biologie, which he then started (187o-71), and which has since become so renowned. Investiga tions on other branches of bacteriology soon followed, among which "Organismen der Pockenlymphe" (1872) and "Untersuch ungen uber Bacterien" (1872-1875) are most important, and laid the foundations of the science. Cohn brought out and helped R. Koch in publishing his celebrated paper on Anthrax (1876), the first clearly worked out case of a bacterial disease.
Among his most striking discoveries may be mentioned the na ture of Zoogloea, the formation and germination of true spores -which he observed for the first time, and which he himself dis covered in Bacillus subtilis-and their resistance to high tempera tures, and the bearing of this on the fallacious experiments sup posed to support abiogenesis ; as well as works on the bacteria of air and water, the significance of the bright sulphur granules in sulphur bacteria, and of the iron oxide deposited in the walls of Crenothrix. His discoveries in these and in other departments attest his acute observation and reasoning powers. Cohn had clear perceptions of the important bearings of Mycology and Bacteriol ogy in infective diseases, as shown by his studies in insect-killing fungi, microscopic analysis of water, etc. He was a foreign mem ber of the Royal Society and of the Linnean Society, and received the gold medal of the latter in 1895. He died at Breslau on June 25, 1898.
Lists of his papers will be found in the Catalogue of Scientific Papers of the Royal Society, and in Ber. d. d. bot. Gesellsch., vol. xvii. p. 196 (1899) ; see also P. Cohn, Ferdinand Cohn, with Life by F. Rosen.