Ferdinand Julius Cohn was a German botanist and bacteriologist whose efforts in developing bacteriology as a separate science earned him the title “father of modern bacteriology.”
Background
Cohn was born on January 24, 1828, in Breslau (now Wrocław, Poland), to impoverished young parents in Breslau’s Jewish ghetto. His father, Issak, soon achieved success as a merchant and was able to nourish and encourage Ferdinand’s precocious talents. His childhood was troubled by poor health and weak hearing.
Education
It is said that Cohn could read at age two and that he was familiar with the basic doctrines of natural history by age three. He first attended school at age four; and after entering the Breslau Gymnasium in 1835, he advanced rapidly until, at age ten or eleven, a hearing defect began to slow his incredible pace. A shy, studious, and sensitive child, Cohn suffered from an acute sense of physical and emotional retardation that he did not begin to overcome until his last year in the Gymnasium.
In 1842 he entered the philosophical faculty of the University of Breslau (now University of Wrocław), uncertain as to his career goals although he was inclined to the professions. In time, botany became his chief interest, mainly because of the influence of the Breslau professors Heinrich Goeppert and Christian Nees von Esenbeck. Although Cohn grew up in a period of partial liberalization of earlier restrictions on Jews, he was nevertheless barred as a Jew from the degree examinations at Breslau. After his petition for removal of the restriction was denied by the government, Cohn went to the University of Berlin in October 1846; there he received his doctorate in botany on 13 November 1847, at the age of nineteen. At Berlin he was stimulated by the teaching of Eilhard Mitscherlich, Karl Kunth, Johannes Müller, and especially Christian Ehrenberg, who introduced him to the study of microscopic animals as well as plants.
Career
In 1849 Cohn returned to Breslau (now Wrocław, Poland), which remained his home for the rest of his life. In 1850 Cohn was recognized as a Privatdozent at the University of Breslau (now University of Wrocław). He was appointed extraordinary professor of botany there in 1859 and ordinary professor in 1872. He married Pauline Reichenbach in 1867.
Cohn began his career in the midst of an intellectual revolution in botany produced by Matthias Schleiden’s cell theory and Hugo von Mohl’s description of protoplasm in the plant cell. In these circumstances Cohn’s decision to focus his interests on the lowest plants, especially unicellular algae, probably resulted in part from his conviction of the value of cellular studies and his belief that the best way to gain insight into the cellular processes of higher organisms was to begin by carefully studying the cellular processes of the simplest organisms. In 1848 Cohn’s former teacher Goeppert asked Cohn to devote himself especially to algae, in the hope that he would then contribute to a projected flora of the cryptogamous plants of Silesia. By the time this projected flora began to be published in 1876, it was Cohn, not Goeppert, who directed the work and edited the first two volumes.
In the meantime Cohn had gained early fame for his work of 1850 on the unicellular alga Protococcus pluvialis, especially for his novel suggestion that the protoplasm in plants and the “sarcode” in animals were “if not identical, at least highly analogous formations.” This represented an important step toward the belief that the basic attributes of all life were to be sought in a single substance called protoplasm. This protoplasmic theory of life received its classic expression in a paper published in 1861 by the German histologist Max Schultze, and only near the end of the century did this conception of protoplasm as the unitary substance of life break down, to be replaced by the notion that protoplasm is a dynamic emulsion of several distinct substances and therefore has only a morphological significance.
Cohn’s work on P. pluvialis also marked an important advance toward a redefinition of the cell and toward a recognition that Schleiden had placed too much emphasis on the cellulose cell wall. By suggesting, like Braun, that there existed primordial plant cells (motile swarm cells) devoid of cellulose walls, Cohn confirmed and expanded the suggestions of Nägeli, Mohl, Braun, and others that the essential constituent of the cell was its protoplasmic contents.
In 1854 Cohn incorporated much of his earlier work into a large treatise on the developmental history of microscopic algae and fungi. His main conclusion - that algae and fungi should be united into one class - was soon discredited, but the treatise contained much of lasting value. In his article of 1855 on the unicellular alga Sphaeroplea annulina, Cohn contributed importantly to developing notions about the sexuality of the algae. Following the demonstration of sexuality in the brown marine alga Fucus by Thuret (1855) and in the freshwater alga Vaucheria by Pringsheim (1855), Cohn extended these conclusions to Sphaeroplea, another freshwater alga.
In the decade from 1856 to 1866, Cohn published an important paper on the contractile tissues of plants (1860), reemphasizing his conviction that contractility was not confined to animal tissue, and a model experimental study on phototropism in microscopic organisms (1865). But even though he remained an active investigator during this decade, Cohn’s contributions to the botanical literature were less original and less important than his earlier work. He devoted much of his time to consolidating his earlier work and to nonresearch matters. At the Schlesische Gesellschaft fur Vaterlandische Kultur, of which he had been a member since 1852, Cohn accepted the chairmanship of the botanical section in 1856 and remained active in this post for the next thirty years.
His most important activity from 1856 to 1866 was his agitation, eventually successful, for an institute of plant physiology. This had been a matter of top priority for Cohn since 1847, when he had defended at Berlin the thesis that Germany needed institutes of plant physiology. In 1866 the Breslau authorities finally acceded to Cohn’s long-standing request and acquired a nearby building that had once been a prison. In these inauspicious surroundings Cohn founded the first institute for plant physiology in the world, and soon launched the second great creative period of his career.
One of the earliest apparatuses installed in Cohn’s institute was a small, simple marine aquarium that yielded material for much of his later work. In 1866 he reported on some new infusoria that he had found in this aquarium and revealed his method of cultivating marine plants. About 1870 Cohn turned his attention primarily to bacteria, and it is for his researches in this area that he is best known. In 1870 he founded a journal, Beiträge zur Biologie der Pflanzen, designed primarily to publish the work that came out of his institute. In this journal appeared the founding papers of modern bacteriology. Cohn initiated the movement with his classic work of 1872. Although later regarded as a classic work of science, Cohn’s treatise of 1872 did not immediately convince everyone. In particular, its conclusions were disputed by those, such as Theodor Billroth, who believed that the various external shapes of bacteria did not really correspond to distinct genera and species but were merely various stages in the developmental history of a single plant form. In his work on Coccobacteria septica (1874) Billroth argued that all bacteria belonged to a single plant species whose various shapes appeared mainly in response to altered environmental circumstances. The various forms were, he argued, ultimately convertible one into the other.
In 1875, in the second of his “Researches on Bacteria,” Cohn defended his earlier work and rejected Billroth’s conclusions. In 1876 Cohn discussed in greater detail the implications of his discovery of thermoresistant endospores in Bacillus subtilis for the controversy over spontaneous generation. He believed that his discovery could at last explain the well-known anomalies presented by boiled infusions of hay and turnip - cheese. In all such infusions, Cohn demonstrated, it was the thermoresistant spores of Bacillus subtilis which had caused the difficulties. He showed that boiled hay infusions, like boiled infusions of turnip and cheese, contained bacillus spores, which were capable of surviving strong heat and then germinating to form new bacilli.
Cohn’s paper of 1876 was his last important contribution to bacteriology, except insofar as his direct and indirect influence is revealed in the subsequent “Researches on Bacteria” that appeared in his journal5. He had made four contributions of fundamental importance to bacteriology: his system of classification (1872); his discovery of spores (1875); his discussion of the implication of his discovery of spores for the question of spontaneous generation (1876); and his Beiträge zur Biologie der Pflanzen, in which the founding papers of modern bacteriology appeared.
Besides his scientific monographs and treatises, Cohn published many popular lectures and the widely read book Die Pflanze (1882), which was graced with history, biographical notes, and Goethe-inspired poetry and was credited with making innumerable converts to botany.
In 1887 the University of Breslau provided him with a new institute of plant physiology in the Breslau botanical gardens. Cohn held an honorary doctorate from the faculty of medicine at the University of Tubingen and was named a corresponding member of the Accademia dei Lincei in Rome, the Institut de France in Paris, and the Royal Society of London. In 1885 he was awarded the Leeuwenhoek Gold Medal and in 1895 the Gold Medal of the Linnean Society.
Politics
When revolution rocked Berlin in March 1848, Cohn was in passionate sympathy with the revolutionaries. Although he did not himself assume an active role, Cohn’s academic career may well have suffered because of his political opinions, as well as because he was a Jew.
Views
In 1867 Cohn suggested that since the red algae of the Oscillaria family could survive in primitive environments fatal to other plants, they must have been the first inhabitants of earth and the plants from which the rest of the plant world evolved. This led him to attempt a classification of previously neglected lower plants. Although not entirely successful, it was a pioneering attempt to base a classificatory system on Darwinian evolutionary principles.
Cohn also showed, by researches on the nutrition of Bacterium termo, that bacteria were like green plants in that they obtained their nitrogen from simple ammonia compounds but unlike green plants in that they were unable to take their carbon from carbonic acid, requiring instead carbohydrates and their derivatives. Arguing that putrefaction was a chemical process excited by the growth of Bacterium termo, Cohn also maintained that there was a clear distinction between these bacteria of putrefaction and pathogenic bacteria.
Then, in a long series of experiments, Cohn proved that a temperature of 80°C. effectively destroyed the life of virtually all bacteria and prevented their development in an organic infusion; he admitted, however, that some doubt remained regarding Bacillus subtilis, the bacteria of butyric fermentation, which were more resistant to heat than B. termo.