30 April 2004

The Leduc Plants: An Example of Amusing Science from the History of Science

Carlos Eduardo Sierra C.

National University of Colombia, Processes and Energy School
Member of the British Society for the History of Science

Literature, History of Science and Amusing Science

In chapter 3 of Doktor Faustus, Thomas Mann's well-known novel, we find a wonderful and fascinating description of two phenomena that suggest at first sight a connection between life processes and physicochemical principles: osmotic growing and voracious drop. In this chapter Mann seems to reflect on the heated discussion of a quarter of a century earlier concerning the claims of the supporters of plasmogenesis. This was  the hypothesis that living organisms could arise from nonliving substances by means of the exclusive participation of physicochemical causes.  
     
In the history of science, the plasmogenesis affair has been largely forgotten, at least in view of the number of information sources about the matter. In fact, if we browse the Web with the assistance of a search tool such as Copernic Agent Professional, we can find only one or two results for the term "Leduc plants" in Spanish and in English, even with the Copernic maximal search capacity. The word "plasmogenesis" yields eight results. This is difficult to believe, since the Leduc plants are the central episode of this astonishing theme, a founding myth so to speak.

When reviewing the usual sources on osmosis for their use in physical chemistry courses and lectures, the absence of plasmogenesis is upsetting. Therefore, we must focus our attention on information sources of a different nature. Furthermore, when we perform experiments and demonstrations to illustrate various reactions and principles to classes, workshops and lectures, it is good to keep in mind the importance to the audience of the proverbial Aristotelian maxim of teaching with amusement, which the Leduc plants episode certainly provides.

How to Do Experiments with Leduc Plants

Thomas Mann, in Docktor Faustus, describes the realization of this kind of experiment by the use of potassium salicylate in an aqueous solution and "diverse crystals," which have been mixed with sand at the bottom of a glass vessel. However, the eminent German writer does not provide key details about quantities and concentrations in relation to the necessary chemicals. Therefore, this evocative narrative is not very useful for executing these experiments. Nevertheless, we can find in other sources the necessary details. For instance, the Catholic intelligentsia produced various publications about Leduc plants because of the philosophical background that characterized the debate about plasmogenesis. Let us consider now two significant examples.

Brother Daniel, an outstanding Colombian scientist, wrote a book for high school students about botany and zoology. This book, which was published in 1952, gives the historical, philosophical and physicochemical circumstances connected with the Leduc plants affair. Based on this book and my own experience, I offer the following experimental procedure to make Leduc plants:

(a) It is first necessary to form small granules composed of copper sulfate, table sugar and sufficient water to form a paste. In a scale, weigh 1 gram of copper sulfate and 2 grams of table sugar. Next, pulverize  these ingredients separately in a mortar so they can be well mixed together. Add distilled water a drop at a time with the aid of a dropper until a very viscous paste is formed. While wearing a thin latex or rubber glove, place a small quantity of this paste between the forefinger and the thumb and roll the paste to form a small ball of the desirable size. As soon as the ball is formed, place it on an enamel or plastic surface to dry as shown in Figure 1. Repeat this procedure to form additional balls from the remaining paste. The granules should have a diameter of at least 1-2 millimeters. Larger granules of 5-7 millimeters can be used to form larger Leduc plants.

(b) It is next necessary to prepare a solution of potassium ferrocyanide (2-4 % by weight), sodium chloride (1-10 % by weight) and gelatin (1-4 % by weight).

Caution: During this activity, use caution to avoid spilling or splashing the liquids into your eyes or onto skin or clothing. Be sure to wear rubber gloves while working with the sulfates and the potassium ferrocyanide because of their relative toxicity.

Begin by pouring 300 cubic centimeters of distilled water in a 500 cc beaker. Heat the water until it reaches a temperature of approximately 70 ºC. Next, add little by little 3 grams of gelatin, stirring each small portion with a stirring rod until it is dissolved. Now, add 3 grams of sodium chloride (table salt) and shake until it is completely dissolved. Finally, add 6 grams of potassium ferrocyanide and shake the solution until it is dissolved. Allow the solution to cool to room temperature by placing the beaker on a wood surface.

(c) Fill a test tube with the solution ("nutrient medium"), put it in a test tube holder, and add a granule ("seed") to the solution (see Figures 2 and 3). Instead of a test tube, you can use a glass flower vase (see Figure 4). After a few minutes, the "seeds" will begin to "germinate.". Fine stalks, resembling rice, are formed, which are subdivided and constitute exuberant and flexible "vegetation".

With regard to the previous experimental procedure, it is important to note that the purity of the reagents is generally not essential to producing good Leduc plantlike structures. There are exceptions, which we mention below.  We can use chemicals of commercial or USP pureness grade, which significantly reduces the costs of these experiments. Thus, this experiment is within reach of everybody.

However, the experimental procedure described here is not the only one, since the historical literature describes a broad panoply of other versions. A classical essay written in 1921 by Father Jaume Pujiula, the Spanish Jesuit priest and scientist, may be  the best source in this respect. In this essay, Father Pujiula examines carefully the various Leduc experiments and discusses their epistemological consequences. In any case, these experimental variants rely on the following repertoire:

(a) Use zinc or ferric sulfate instead of copper sulfate.

(b) Make granules of potassium ferrocyanide and table sugar and sow them in a "nutrient medium" composed by gelatin, copper (or zinc or ferric) sulfate and sodium chloride in aqueous solution.

(c) Employ directly as "seeds" some crystals of the aforesaid salts (sulfates or potassium ferrocyanide) instead of the granules we have described above.

As noted above in regard to these substances and their handling, it is very important to use caution and top avoid spilling or splashing the liquids into ones eyes or onto skin or clothing. Be sure to wear rubber gloves while working with the sulfates and the potassium ferrocyanide because of their relative toxicity.

In general, the variety of forms that can be obtained and their production mode depend on these factors: (a) concentration of gelatin, (b) concentration of potassium ferrocyanide, (c) quality of gelatin and its coagulation rate, (d) size of the granules, and (e) temperature. These factors are clues the reader should have in mind while performing his or her own experiments in accordance with the aforementioned Aristotelian maxim of teaching with amusement. After all, knowledge can only produce happiness according to Baruch Spinoza's keen thought.

How can we explain the associated phenomena in all these experiments? Father Pujiula's paper offers an explanation in the following terms. Once a granule is placed in the "nutrient medium," it is immediately covered by a film of cupric ferrocyanide (in the case of the version with copper sulfate), which is a semipermeable membrane that will allow water to pass but not the sugar. This phenomenon is osmosis. Therefore, a strong osmotic pressure is generated within the granule. The effect of the pressure is to distend the membrane until it breaks, and a part of the content is poured out.

At this point, the potassium ferrocyanide of the "nutrient medium" reacts with the copper sulfate of the granule, and the membrane of cupric ferrocyanide is regenerated. Then, the osmotic pressure increases again in the granule. The membrane is distended and broken, and the fractures are closed up again. This liberates the reagents to regenerate the process, which will repeat itself while there are substances that can react. In brief, we have here an example of osmotic growth. Of course, these phenomena are not living processes. Why?

We began this paper with Thomas Mann, and now we shall conclude with him. In "The Magic Mountain," he writes the following words: Gems, diadems, diamond brooches -the most skilled jeweler could have created no richer and more delicate work… and in all the myriads of enchanting stars with their secret miniature splendor, too small for the naked eye of man to see, was not one like unto another.

Mann's words reflect the philosophical marvel accompanying the spectacle of the contemplation and understanding of nature. And it is in this perspective we must place the Leduc plants as a source of intellectual fruition in harmony with rigorous thinking.

The Leduc plants episode ended officially in 1905, when Robert Dolfus, a young man only twenty-one years old, sent a note by way of Gaston Bonier to the French Academy of Sciences that explained the osmotic principle underlying the growth of the Leduc plants. Thus, the pseudoscientific doctrine of spontaneous generation came to its decisive end.

Nevertheless, if we browse the Web, we shall find information on the supposed validity of spontaneous generation, just as we can also find information about the supposed possibility of perpetual motion. In short, it can be said that these examples suggest a mistaken comprehension of natural laws.

Nevertheless, we can always benefit from the Leduc plants by using them to illustrate osmotic phenomena in classes, lectures and workshops, accompanied by an amusing dimension resulting from its historical background.

References

Hermano Daniel, Sinopsis de biología general: Botánica y zoología aplicadas. (Bedout, Medellín, 1952), p.32-33.

Manfred Eigen, 'How Does Information Originate? Principles of Biological Self-Organization' in For Ilya Prigogine, ed. Stuart A. Rice, (John Wiley & Sons, New York, 1978), p.211-262.

Jaume Pujiula, 'Plasmogenia' in Enciclopedia Universal Ilustrada Europeo-Americana, Tomo XLV, (Hijos de J. Espasa, Barcelona, 1921), p.454-463. ( www.valencia.edu/~orilife/Autors/Pujiula.htm ).