Cohn, Ferdinand Julius

(b. Breslau, Lower Silesia [now Wroclaw, Poland], 24 January 1828; d. Breslau, 25 June 1898),

botany, bacteriology.

Cohn was born 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. 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, 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. 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. In 1849 he returned to Breslau, which remained his home for the rest of his life. In 1850 Cohn was recognized as a Privatdozent at the University of Breslau. 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.” “The motile forms of P. pluvialis, with their long protoplasmic flagella, reminded Cohn of such flagellated infusoria as Euglena and the Monades. From this base, he argued against attempts to distinguish animals from plants on the basis that the former possessed differentiated organ systems or a contractile substance peculiar to themselves. Dujardin and Siebold had already shown that infusoria and rhizopods—although classified as animals—did not contain differentiated organ systems; and Cohn now suggested that the optical, chemical, and physical properties which Dujardin had ascribed to animal “sarcode” were also possessed by plant protoplasm.

Several others were working toward this same conclusion, particularly Alexander Braun, who had implied the identity of plant and animal protoplasm in a work sent to Cohn four months before the latter’s first communication on P. pluvialis.¹ But Cohn, who was more familiar with the zoological literature, was the first to draw explicit attention to the identity between the contractile contents of plant and animal cells. 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,² 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. Of particular interest was the section on bacteria—then called Vibrionia. Up to this time, no one had questioned the animal nature of the Vibrionia, but Cohn argued that they were plants by virtue of their close relationship to known algae. The Vibrionia had been considered animals primarily because of their active, apparently voluntary, movement; but Cohn pointed out that the ciliated swarm cells of algae and fungi performed similar movements. He also suggested that bacteria followed the same developmental course as algae, demonstrating this by comparing the development of Bacterium termo Dujardin with the alga Palmella. Cohn did not claim definitive observations for the other genera of Vibrionia, but suggested that the larger bacteria seemed also to belong to the plant kingdom and seemed to display an especially close relationship to the Oscillaria.

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. He observed in Sphaeroplea the formation of spermatozoa and followed their progress all the way to the egg, although he was unable to see them actually penetrate it. In 1856 Cohn demonstrated another case of sexuality in algae—this time in Volvox globator, a motile form; previously sexuality had been demonstrated only in filamentous algae.

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. 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.

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, of which William Bulloch wrote that “its perusal makes one feel like passing from ancient history to modern times.” In this treatise Cohn tried to bring order out of the chaos caused by the use (especially by Pasteur) of vague and arbitrary names for bacteria and by the frequent introduction of new terms. He defined bacteria as “chlorophyll-free cells of spherical, oblong, or cylindrical form, sometimes twisted or bent, which multiply exclusively by transverse division and occur either isolated or in cell families” and distinguished four groups of bacteria on the basis of their constancy of external form: (1) Sphaerobacteria (round), (2) Microbacteria (short rods or cylinders), (3) Desmobacteria (longer rods or threads), and (4) Spirobacteria (screw or spiral). Under these four groups Cohn recognized six genera of bacteria, with at least one genus belonging to each group. He repeated his conclusion of 1854 that bacteria belong to the plant kingdom by virtue of their affinity with well-known algae and suggested—in opposition to Hallier—that there was no genetic relationship between bacteria and the fungi with which they often appeared.

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. His experiments on the effect of low temperatures showed that although bacteria were rendered torpid by long exposure to freezing temperatures, they were not killed and regained their former vitality with the return of higher temperatures. He suggested, finally, that Pasteur’s difficulty in dealing with the French supporters of equivocal generation resulted primarily from the fact that certain conditions relating to bacteria remained poorly understood.

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. He pointed out that he had based his classificatory scheme on external form primarily because he had found that certain characteristic physiological phenomena—especially specific fermentative activities—were associated with specific and apparently constant forms of bacteria. He did not insist that distinctions based on the external form and fermentative activity of bacteria were necessarily natural divisions; but he strongly defended his method of focusing on the lower and simpler life conditions of the fermentative organisms, since the question was one of ascertaining the general biological relationships of bacteria.

In one long and interesting section of this treatise of 1875, Cohn discussed Bastian’s controversial experiments on turnip-cheese infusions. Bastian had found that bacteria could appear in such infusions even after ten minutes of boiling in a sealed flask. Assuming that the boiling destroyed all organisms previously living in the flask, Bastian concluded that living organisms could originate through abiogenesis (a sophisticated form of spontaneous generation).

When Cohn repeated Bastian’s experiments, he obtained the same results but did not accept Bastian’s conclusion. He argued that there might be a special developmental stage or germ that survived the boiling. He showed that the bacteria which appeared after boiling in cheese infusions were not the common putrefactive bacteria (B. termo) but, rather, bacillus rods or threads, which he identified as Bacillus subtilis (Pasteur’s butyric ferment), whose resistance to temperature he had already mentioned in his work of 1872. Close observation of the bacilli in boiled cheese infusions revealed that after a short time many of them swelled at one end and became filled with “oval or roundish, strongly refractive little bodies” which multiplied continuously. Cohn asserted his conviction that these little bodies represented a stage in the life cycle of the bacilli and proposed as highly probable that they were “real spores, from which new Bacilli may develop.” Since it was known that other spores were thermoresistant, it seemed likely that it was the spores in Bacillus subtilis which survived the boiling and germinated to form bacteria in Bastian’s boiled and sealed turnip-cheese infusions.

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 also showed that the complete growth and development of bacilli, and especially the formation of their spores, depended on the presence of air. When air was excluded, the activities of the bacilli induced butyric fermentation. Since normal putrefaction took place only in the presence of B. termo, while butyric fermentation took place only in the presence of Bacillus subtilis, Cohn felt that this work gave new support to his thesis that there existed distinct, independent, and incontrovertible genera of bacteria with different courses of development, different biological properties, and different fermentative activities.

In another section of this treatise of 1876, Cohn described the results of a series of experiments designed to reveal the effects of temperatures of less than 100°C. on the development of bacilli in hay infusions. He reported that bacilli, unlike any other bacteria or fungi in hay infusions, were capable of normal activity at temperatures up to 50°C. Between 50° and 55°, all multiplication and development of mature bacillary threads ceased, as did spore formation; but any spores already formed survived and retained their ability to germinate. Hay infusions were generally sterilized after twenty-four hours of heating at 60°C., but individual bacillary spores retained the ability to germinate even after three to four days of heating at 70° to 80°C.

Cohn promised a more exact determination of the temperature limits within which the development of bacilli could take place, but this project was forestalled by John Tyndall’s important researches on the sterilization of hay infusions by discontinuous heating. Utilizing a superior experimental design, Tyndall carried the argument against spontaneous generation to a new level of completeness; but is should be remembered that in the crucial case of the hay bacillus (Bacillus subtilis) he had been forearmed with the results of Cohn’s work of 1876³. Cohn and Tyndall together contributed as much as Pasteur to the final overthrow of the old doctrine of spontaneous generation.

At the end of his paper of 1876, Cohn referred to the pathogenic significance of the bacilli that appeared in the blood of animals and men afflicted with anthrax; he then introduced Robert Koch’s classic paper on Bacillus anthracis, which followed immediately. Cohn stated his conviction that there could be no uncertainty about Koch’s results. After Koch became famous for his contributions to bacteriology, a myth developed, especially in Breslau, to the effect that Koch had been a student of Cohn’s and that his ideas owed much to Cohn’s influence. In 1890 Cohn himself clarified this situation in an accurate and characteristically generous manner by pointing out, as he had when he introduced Koch’s paper in 1876, that Koch had come to Cohn’s institute at Breslau only to demonstrate results which he had already reached on his own and to ask Cohn and his colleagues for their judgment of his work. Cohn’s role was essentially that of stimulating and encouraging Koch’s work and of providing a place for its publication.⁴

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: (1) his system of classification (1872), (2) his discovery of spores (1875), (3) his discussion of the implication of his discovery of spores for the question of spontaneous generation (1876), and (4) 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.

NOTES

1. Braun implied the identity in his Betrachtungen über the Verjüngung in der Natur, besonders in dem Leben und Entwicklung der Pflanzen (Leipzig, 1851 [preface dated 1850]). Braun sent a copy of this work to Cohn on 10 May 1850, and although Cohn was already at work on Protococcus pluvialis, he did not present his first communication on it until September of that year (see Ferdinand Cohn, Blätter der Erinnerung, pp. 83–87).

2. Max Schultze, “Ueber Muskelkorperchen and dass was man eine Zelle zu nennen habe,” in Archiv für Anatomie, Physiologie and wissenschaftliche Medicin (1861), 1–27.

3. See John Tyndall, “Further Researches on the Deportment and Vital Resistance of Putrefactive and Infective Organisms, From a Physical Point of View,” in Philosophical Transactions of the Royal Society, 167 (1877), 149–206. On p. 152 Tyndall reports that in the autumn of 1876 Cohn “placed in my hands” his treatise of 1876. See also Tyndall’s “on Heat as a Germicide When Discontinuously Applied,” in Proceedings of the Royal Society (London),25 (1887), 569–570.

4. Cohn clarified his relationship with Koch in the newspaper Breslauer Zeitung (17 Dec. 1890).

5. Between 1872 and 1885 a dozen “Resarches on Bacteria” were published in Cohn’s Beiträge. Three of these he wrote himself, one he coauthored, and most of the rest were contributed by his students.

BIBLIOGRAPHY

I. Original Works. Cohn’s writings discussed in the text are “Nachträge zur Naturgeschichte des Protococcus pluvialis Kützing” [1850], in Nova acta physico-medica Academiate Caesareae Leopoldino Carolinae germanicae naturae curiosorum, 22 (for 1847), 605–764; Untersuchungen uber die Entwicklungsgeschichte der mikroskopischen Algen and Pilze,” ibid., 24 (1854), 103–256; “Ueher die Fortpflanzung von Sphaeroplea annulina,” in Bericht über die zur Bekanniomachung geeigneten Verhandlungen der K. Preussischen Akademie der Wissenschaften zu Berlin (1855), 335–351; “Beobachtungen über den Ban and die Fortpflanzung von Volvox globator,” Ubersicht der Arbeiten and Veranderungen der Schlesischen Gesllschaft fur vaterlandsiche Kultur (Breslau) (1856), 77–83; “Contractile Gewebe im Pflanzenreiche” [1860], in Jahresbericht des Akademischen naturwissenschaftilichen vereins zu Breslau (Ahhandlungen) (1861) 1–48; “uber die Gesetze der Bewegung der mikroscopischen Pflanzen and Their unter Einfluss des Lichtes,” in Bericht über die Versammlung der Deutshen Naturforscher and Bericht Aerzte, 40 (1865), 219–222; “Neue Infusorien im See aquarium” [1865], in Zeitschrift für wissenschafliche Zoologie, 16 (1866), 253–302; “Beiträge zur Physiologic der Phycochromaceen and Florideen,” in Archiv für mikroskopische Anatomie and Entwicklungsmechanik, 3 (1867), 1–60; “Untersuchungen über Bacterien,” in Beiträge zur Biologie der Pflanzen, 1 , no. 2 (1872). 127–224; “Untersuchungen uber Bacterien, II, “ibid no. 3 (1875), 141–207; “Untersuchungen uber Bacterien, IV. 249–276; and ibid2 no. 2 (1876), 249–276; and Die Pflanze: Vorträge arts dent Gebiete der Botanik(Breslau, 1882, 1897).

At least one paper not discussed in the text deserves special mention: “Empusa muscae and die Krankheit der Stubenfliegen: Ein Beitrag zur Lehre von den durch parasitische Pilze charakterisirten Epidemieen,” in Nova acta physico-medica Academiae Caesareae Leopoldino Carolinae germanicae naturae curiosorum, 25 (1855), 299–360, which is noteworthy as the first careful study of a disease in animals (the housefly) caused by a fungus. Cohn published nearly 200 papers and books: almost all of them can be located by judicious use of these three bibliographies:(1) the bibliography in Ferdinand Cohn, Blätter der Errinerung, pp.261–266 (see below); (2) Royal society Catalogue of Scientific papers, II, 8–10; VII, 413–414; IX, 547–548; XIV, 294; (3) British Museum General Catalogue of Printed Books, XLI (1966), 322–323.

II. Secondary Literature. The basic source for Cohn’s life and work is Ferdinand Cohn, Blätter der Errinerung (Breslau, 1901), collected by his wife Pauline Cohn, with contributions by Felix Rosen. This truly remarkable book is filled with intimate insights into Cohn, his work, and his times and is based upon his boyhood diary (discontinued in 1852), his family letters, scientific correspondence, congratulatory notes, and descriptions of his travels throughout Europe. In his last few months Cohn began an autobiography, but he had completed only a few pages at the time of his death.

Fairly detailed accounts of Cohn’s life and work are also given by Felix Rosen, in Berichte der Deutshen botanischen Gesellschaft, 17 (1899), 172–201; and also by C. Mez, in Biographisches Jahrbuch and Deutscher Nekrolog III,(1900), 284–296. For still other biographical sketches, see Neue Deutsche Biographie, III, (1957), 313–314; Münchener medicinische Wochenschrift, 45 , pt.2 (1898), 1005–1007; and the references given in the Royal Society Catalogue of Scientific Papers, XIV, 294.

For Cohn’s place in the history of botany, see Julius von Sachs, Geschichte der Botanik vom 16 Jahrhundert bis 1860 (Munich, 1875), pp. 225,228, 478; and John R. Baker, “The Cell-Theory: A Restatement, History, and Critique, Part II,” in Quarterly Journal of Microscopical Science, 90 (1949), 87–108 (94–96). For his place in the history of bacteriology, see William Bulloch, The History of Bacteriology (London, 1938), pp. 106, 113, 116–119, 150, 174–177, 187–188, 192–195, 198, 200–203, 207–210, 213–214, 217–218, 296, 319, 328–330, 358.

A brief sketch of Cohn’s life and an assessment of his bacteriological researches can be found in Morris C. Leikind’s introduction to Cohn’s “Bacteria, the Smallest of Living Organisms,” [1872], Charles S. Dolley, trans. [1881], in Bulletin of the History of Medicine, 7 (1939), 49–92, to which is appended a reprint of the bibliography of Cohn’s works found in Blätter der Errinerung.

Gerald L. Geison

Ferdinand Cohn

Considered to be the father of modern bacteriology, Ferdinand Cohn (1828-1898) began his studies as a botanist and ultimately made discoveries which led to the creation of a new field of study. He was the first scientist who believed that bacteria should be classified as plants. His contributions include a system of classification and the discovery of spores, with its implication for spontaneous generation. His journal, Beitrage zur Biologie der Pflanzen, contained the first essays on modern bacteriology.

Ferdinand Julius Cohn was born in the German Jewish ghetto of Breslau, Silesia (now Wroclaw, Poland), on January 24, 1828. His father, Issak Cohn, became a successful merchant and was able to support his son's intellectual talents. A child prodigy, Cohn could read at the age of two, began school at four, and entered the Breslau Gymnasium (high school) in 1835. A hearing defect slowed his progress in school and contributed to his shyness and sensitivity as a young man. Still, in 1842, he was able to enter the University of Breslau. Influenced by professors Heinrich Goeppert and Christian Nees von Esenbeck, Cohn developed an interest in botany. Because he was Jewish, he was barred from taking the degree examinations at Breslau. He applied for an exemption from this restriction, but was refused. Cohn went to Berlin in 1846, and studied under Eilhard Mitscherlich, Karl Kunth, Johannes Muller, and Christian Ehrenberg, who introduced him to the study of microscopic animals. On November 13, 1848, at the age of nineteen, he received his doctorate in botany. His thesis dealt with the concept that each country must establish institutes for plant physiology.

Returned to Breslau

In March 1848, Berlin was engulfed in a rebellion. Cohn supported the revolutionaries in spirit, although he did not actively participate. Because of his political opinions, and possibly because he was Jewish, Cohn was refused a teaching position in Berlin. He returned to Breslau in 1849 and obtained a teaching position at the University of Breslau, where he would remain for the rest of his life. He was appointed associate professor of botany in 1859 and married Pauline Reichenbach eight years later. In 1872, Cohn was became a full professor.

The mid-nineteenth century was an exciting time for botanists. Scientists such as Matthias Schleiden investigated cell theory and Hugo von Mohl described the protoplasm in a plant cell. Cohn decided to study the smallest organisms with a particular focus on protoplasm. Through his work on the unicellular algae, Protococcus pluvialis, he determined that the protoplasm in plants and the "sarcode" in animals were very similar. Cohn concluded that protoplasm contained the basic characteristics of all life. This view gained him a considerable amount of fame. Only at the end of the nineteenth century did scientists understand that protoplasm was a dynamic emulsion that could be further broken down into several different substances.

During this period Cohn, at the request of his former teacher Goeppert, did an extensive study of algae. By 1854, he had put together a work on the developmental history of microscopic algae and fungi. His conclusion, that algae and fungi belong to one class, turned out to be false. However, the section of the work which had lasting value dealt with a bacterium called Vibronia. Cohn insisted that Vibronia were plants because of their similarity to the development of algae. Vibronia had long been thought to be animals because they propelled themselves quickly by cilia or long tendrils. Cohn recognized that Vibronia were similar, yet different from fungi and algae. Also, they developed in much the same manner as algae. In an article on the unicellular algae, Sphaeroplea annulina, published in 1855, Cohn explored the sexuality of the algae, following the spermatozoa all the way to the egg.

Founded Institute for Plant Research

One of Cohn's top priorities for twenty years had been to create an institute of plant physiology. In 1866, the university obtained an old building that had been a prison and allowed him to develop the first institute for plant physiology in the world. Cohn was the director of the institute from the time it opened in 1869 until his death. Using a small marine aquarium, he cultivated and studied marine plants. Here he drew much of the material for his later work. Because the red algae of the Oscillaria family could survive in primitive environments in which other plants could not, Cohn believed that they must have been the first inhabitants of earth and the first plants. This led him to the classification of lower plants. His system of classification was a pioneering attempt, though not entirely successful

Produced Major Work

In order to publicize the work of his institute, Cohn began a journal, Beitrage zur Biologie der Pflazen, in 1872. This journal contained the first essays on modern bacteriology and provided an outlet for other pioneers in the field to publish their research. In 1872, Cohn published a paper that defined bacteria as "chlorophyll-free cells of spherical oblong, or cylindrical form, sometimes twisted or bent, which multiply exclusively by transverse division and occur either isolated or in cell families." He classified bacteria into four groups, based on their constancy of external form. They included sphaerobacteria (round), microbacteria (short rods or cylinders), desmobacteria (longer rods or threads), and spirobacteria (screw or spiral). Cohn recognized six genera of bacteria, with at least one genus belonging to each group. He reiterated his conclusion of 1854 that bacteria belong to the plant kingdom because of their similarity to algae. He also suggested that there was no genetic relationship between bacteria and the fungi with which they were often grouped. This ground-breaking paper brought order to the new field of bacteriology.

Cohn studied plant nutrition and concluded that bacteria obtained their nitrogen from simple ammonia compounds, much like green plants. However, they were unable to take their carbon from carbonic acid, using carbohydrates and their derivatives instead. He also found that bacteria could be frozen without being killed. When thawed, they then returned to their former state. He also discovered that most bacteria would die if heated to 80 degrees Celsius. Cohn's conclusions were not universally accepted, and he continued to defend his research in subsequent essays published in his journal.

Spores that Survived Boiling

In 1875, Cohn published his second essay on bacteria and defended the theories outlined in his 1872 essay. New material included a long section on Bastian's experiments on turnip-cheese infusions. Bastian discovered that some bacteria survived boiling after ten minutes in a closed flask. Cohn theorized that there might be a special developmental stage or germ that survived the boiling. The bacteria that appeared after boiling in cheese infusions were not the common putrefactive bacteria, (B. terma), but rather, bacillus rods or threads, which he called Bacillus subtilis. After a short time many of them swelled at one end and became filled with oval, strongly refractive little bodies that multiplied continuously. Cohn believed that these bodies represented a stage in the life cycle of the bacilli and suggested that they were "real spores, from which new Bacilli may develop." Since it was known that spores survived high temperature, he concluded that these must also be spores that survived the boiling and then germinated to form bacteria.

Cohn's last important contribution to bacteriology was published in 1876. He proved that thermoresistant endospores in Bacillus subtilis were capable of surviving strong heat and germinating to form new bacilli. There was no spontaneous generation in this process. He also showed that the presence of air was necessary for the formation of these spores. Therefore, distinct genera of bacteria had different courses of development, different biological properties, and different fermentative activities. He also showed that spores that had already formed in heating of hay infusions of less than 100 degrees Celsius survived and retained their ability to develop even after three or four days of heating. After meeting with Robert Koch in April 1876, Cohn supported his paper on Bacillus anthracis. Koch brought his specimens and records and, for three days, showed Cohn his methods and results. Cohn immediately published what he had learned in his journal.

Although he broke no more new ground in the field of bacteriology, Cohn continued to publish monographs and treatises, lectures, and one book Die Pflanzie, in 1882. His book was very popular and contained history, biographical notes, and some poetry, as well as botany. Cohn returned to the study of plant physiology in the last fifteen years of his life. By the time of his death he had published the first three volumes of his Cryptogam-flora of Silesia. In 1887, the University of Breslau built a new institute of plant physiology in the Breslau botanical gardens. In the final years of his life, Cohn received many honors including an honorary doctorate from the faculty of medicine at the University of Tubingen. He was named a corresponding member of the Academia dei Lincei in Rome, the Institut de France in Paris, and the Royal Society of London. He also received the Leeuwenhoek Gold Medal in 1885 and the Gold Medal of the Linnean Society in 1895. Cohn died in Breslau on June 25, 1898, having made major contributions in the new field of bacteriology.

Further Reading

A Biographical Dictionary of Scientists, edited by Trevor I. Williams, John Wiley & Sons, 1974.

Dictionary of Scientific Biography, edited by Charles Coulston Gillispie, Charles Scribner's Sons, 1971.

Shapiro, Michael, The Jewish 100. A Ranking of the Most Influential Jews of All Time, Carol Publishing Group, 1994. □

Cohn, Ferdinand Julius (1828-1898)

German microbiologist

Ferdinand Cohn, a founder of modern microbiology, became the first to recognize and study bacteriology as a separate science. Cohn developed a system for classifying bacteria and discovered the importance of heat-resistant endospores. Additionally, Cohn recognized that both pathogens and nonpathogens could be found in drinking water and spoke of the importance of analyzing drinking water. Finally, Cohn worked with Robert Koch on the development of the etiology of the anthrax bacillus.

Cohn initially began his studies in botany at the University of Breslau in 1844. After being denied entry into the doctoral program in 1846 because of his Jewish heritage, Cohn moved to Berlin. There he completed his doctoral degree in 1847, at the age of 19, on the structure and germination of seeds.

After returning to Breslau in 1849, Cohn was presented with a top of the line microscope from his father. There he studied the cell biology of plants including the growth and division of plant cells, plasma streaming, cell differentiation, and cellular structures. In time, Cohn's studies were redirected toward algae, protozoa , fungi , and bacteria. His efforts on the developmental and sexual cycles of these microorganisms led to important advancements in cell biology.

At that time, bacteriology was an emerging field and although scientists knew that bacteria existed, they had failed to isolate bacteria in pure cultures. Scientists began to name bacteria without regard for someone else that had already observed and named the very same bacteria. Moreover, scientists believed bacteria to be a single species and that variations observed were due to different stages of development. Cohn recognized that bacteria could not be classified as a single species and developed a system for classifying them. He proposed that bacteria could be divided into groups based on whether they had similar development, chemical make-up, or descent. In 1875, he defined bacteria as "chlorophyll-less cells of characteristic shape that multiply by cross division and live as singe cells, filamentous cell chains, or cell aggregates." Eventually he extended his definition to include that "bacteria can be divided into distinct species with typical characteristics, which are transmitted to the following generations when bacteria multiply and that variations exist within each species."

After comprehensive studies of bacteria, Cohn believed that bacteria were related to algae and should thus be classified in the plant kingdom. Additionally, Cohn studied the growth of bacteria and found that in some bacteria organic substances were broken down in the presence of nitrogen. He also claimed that carbon dioxide could not be utilized as a carbon source in bacteria. It was not until 1890 when Sergei N. Winogradsky disproved this statement and discovered autotrophy.

Cohn's initial classification of bacteria consisted of four groups based on shape: Sphaerobacteria (sphere-shaped), Microbacteria (rod-shaped), Desmobacteria (filamentous), and Spirobacteria (screw-like shaped). Of those four groups the genus Micrococcus was classified as Sphaerobacteria, Bacterium was classified as Microbacteria, Bacillus and Vibrio were classified as Desmobacteria, and Spirillum and Spirochaeta were classified as Spirobacteria. Some of the genera could be further divided into subcategories.

Through the studies of Bacillus subtilis Cohn was able to disprove the earlier theory of spontaneous generation. Cohn recognized that some solutions were easily sterilized by heat, requiring only a few minutes of boiling, while other solutions required several hours of boiling. He found that still others, such as hay infusions, could not be sterilized at all. Cohn discovered heat-resistant structures called endospores, not spontaneous generation, were responsible for tainting sterilized cultures. Endospores are not killed in boiling water while the vegetative cells are. It was the heat resistant endospores from which bacteria grew, discounting the old theory of spontaneous generation.

Early on Cohn assisted in diagnosing fungal infections of crops and provided treatment options to the farmers for these plant diseases. Additionally, Cohn recognized that water sources were capable of harboring and transferring infectious diseases to humans. It was Robert Koch who first identified the pathogen that caused cholera in the drinking water; however, Cohn also analyzed the drinking water and found disease and non-disease causing microorganisms. Cohn developed a system for chemical analysis of water and claimed that drinking water should be monitored for microorganisms on a regular basis.

Later when Robert Koch was studying anthrax bacillus, Koch sought the help of Cohn. Cohn realized the importance of studying the disease causing anthrax bacillus and worked with Koch to further investigate the etiology of the bacteria. In 1875, Cohn founded the journal Beitrage zur Biologie der Pflanzen and published Koch's findings on anthrax bacillus in 1877.

See also Water quality; Cell cycle and cell division; History of microbiology

COHN, FERDINAND JULIUS

COHN, FERDINAND JULIUS (1828–1898), German botanist and pioneer bacteriologist. Cohn was born in Breslau, the eldest son of Isaac Cohn, who held the post of Austro-Hungarian consul. He joined the faculty of the University of Breslau in 1851 as a lecturer in botany and in 1872 was appointed professor, the first Jew in Prussia to be granted that rank. Cohn long advocated the establishment of botanical gardens for the rigorous study of functional botany and in 1888 founded the Institute of Plant Physiology. He is generally credited with pioneering the investigation of heat production in plants and encouraging a generation of students to pursue careers in other phases of plant physiology. Cohn's most significant work, however, involved his seminal contribution to the nascent science of bacteriology. He was the first to classify bacteria as plants rather than protozoa, and in 1872 initiated a systematic classification of bacteria based upon their morphological as well as their physiological characteristics. He devised methodological tools which not only afforded a means for assessing biochemical characteristics of bacteria but which also led to the isolation of pure cultures. As the author of the first monograph on bacteria, Untersuchungen ueber die Entwicklungsgeschichte der mikroskopischen Algen und Pilze (1854), he directed the attention of both medical men and biologists to the research and clinical opportunities associated with microbiology at the Botanic Institute in Breslau. He founded and for a long time edited the Beitraege zur Biologie der Pflanzen. Cohn was awarded the Linnaeus Gold Medal. On his 70^th birthday he was made an honorary citizen of Breslau and after his death a monument was erected to his memory.

bibliography:

P. Cohn, Ferdinand Cohn: Blaetter der Erinnerung (1901).

[George H. Fried]