Walter  Dioni  - Cancún, México


In Uruguay 70 years ago the epidermis of the inner side of the scales of the onion bulb was the model used to teach me the configuration of a cell, and I always imagined that it would be reported from years before, on a global scale (in reality, "globally", taking into account the scientific background of those years would mean (in alphabetical order): England, France, Germany, United States).

 Actually the older reference on the epidermis of the foliar sheaths of the onion bulb I now have at my fingertips, via the Internet Archive is dated 1890. 

I made a long review (and a much delayed one!) through "search engines" and in the older texts of Botany, as well as the “Manuals for laboratory practices in Botany”, published on the Internet Archive. I am not complaining. I could see, and I really enjoyed the amazing skill of the artists, who filled those old books with unbelievably beautiful images of plants and its parts, that rival (and many times are better, if we discard the color) many current photos. 



Selecting, I can provide the following data, limiting myself to the last quarter of the 18th century:

 In 1875, R. Bentley gives as an example of epidermis, that of the leaf of Iris, his interest is of course on the cells and stomata, and the Iris is a highly recommended model before and after this date. Bentley published in 1887 (13 years later) a 5th edition of his "A manual of Botany".

He handles properly epidermis and cyclosis, and without using the humble and unknown onion, he shows Fritillaria cells in his fig. 39, describes and picture the cyclosis in potato stem hairs in fig. 40, and the cyclosis and chromoplasts in the aquatic Vallisneria leaf, and illustrates on page 58 the epidermis of Iris, maize (Zea Mais) and "garden Balsam"(Impatiens_balsamina).

Also in 1887, in the English translation of Strasburger’s  "Microscopic Botany", (pag 29, fig 15) the now classical onion only contributes with its roots to the education of botanists. In 1889, Ch.E. Bessey in his “Botany”, uses Fritillaria (p. 3, fig 2) and Tradescantia (p. 12, fig.7) cells in some discussions. In his edition of 1896 of the same text, although citing the onion 7 times he is only interested in the root.

 But E.S.Bastin from the Philadelphia College of Pharmacy, in his "College Botany" (dated 1890, but copyrighted in 1899), on p. 144, Practical excersises 1, and also in their "Laboratory excersises in Botany", (dated 1895, although the copyright is of 1894), on his page 283, gives a very good and extensive description of how to use pedagogically the onion skin to show not only the cells, but even the chemical composition of its walls, illustrating it with a beautiful drawing on the page 289 (plate 38) of the last treatise.

epidermis Bastin 1890

Apparently his books had not much success!

In 1894, Thomas & Dudley, (Plate 1 fig 10, reproduced at right) used as a cell model those in the Tradescantia staminal hairs; the onion is only quoted because of the crystals present in the outer epidermis of the bulb (the tunica)

 They don’t give any image of the calcium oxalate crystals, but only give instructions to chemically test them


tradescantia staminal hair

One portion of a Tradescantia (Rhoeo) spatifolia, staminal hair. DC3 Motic Cámera

tradescantia hair cell

One cell from a staminal hair of Tradescantia (Rhoeo) spatifolia, stained with eosin, from my work on “Eosin as a nuclear stain”  



Three marvelous pictures of a Tradescantia staminal hair  taken though his DIC microscope, were published by Franz  Neidl in:



Another beautiful rendition of this subject is the one by Phil Gates in his blog:



Invented by Georges Nomarski in 1950 the DIC microscope has revolutionized the photomicrography, and become with justice the favorite of now many microscopists.


Interestingly a practical histology handbook mainly dedicated to animal and human histology, Outlines of practical histology, (2nd. ed.) W. Stirling, 1894, uses the "onion skin" as an example of epithelium (fig 57, page 103)

In 1900 Clements & Cutter in his austere "A laboratory manual of High School Botany" recommended cells of several different plants to display various aspects of the same, but never the onion cells. Innovatively, to display the cyclosis they recommended the hairs of the stem of young tomato plants (occasionally other authors recommend (then and now) the cells of the pulp of the fruit of tomato to display cells with coloured vacuoles, or the skin of the same fruit to show chromoplasts)

 Ch.Chamberlain who between 1901 and 1932 published 5 editions of his "Methods in Plant Histology", not even named the onion in 1901, and included it (from the 2nd to the 5th editions) only by its root (on mitosis) and as marginal cyclosis demonstrations; clearly showing that Chara, Elodea and… Tradescantia were more important to him.

 Even in 1911, G.J. Reynolds still uses in his "An introduction to vegetable physiology" Fritillaria (page 5) and Elodea and Tradescantia, (pag.7) images, and in 1921, the cell model shown by Ch.W. Ballard is a drawing, quite rude, of a Tradescantia stem cell, pag.71 (fig. 28). 

 It is clear that pedagogical developments took, at that time, many years to become widespread!

 In the French language, picking only the data most relevant to this topic, we have J-M. A. de Lanessan in 1885, with its Introduction a la Botanique, Ph. van Tieghem in 1892 (who in pg. 13, fig.6 of his Traité de Botanique shows  a cell of Chelidonium sp., similar to the well used Tradescantia cell), and Jules Pavillard in 1901 with his Éléments de biologie végétale, none of which (like his English-speaking colleagues) names at any time the onion epidermis as an example of cellular or tissue structure.

 Seven years later (1908), without I being able to find any intermediate work, although it seems obvious that the practice should have had to be in use in French high schools and Universities from sometime before, Henri Coupin, Henri Jodin and André Dauphine presented their Atlas de Botanique Microscopique, whose Planche I, fig. 1,2,3 show beautiful designs, totally modern, of cells of the onion epidermis, and at fig. 4 a cell of Elodea, according to what is still a standard in elementary biology courses, accompanied by a working protocol that seems written for a modern young microscopist.

In 1921 J.E. Peabody, and A.E. Hunt, published their “Elementary biology, plant, animal, human”. At p. 27 delineate their Lab.Study No. 21, which recommended the use of onion skin (without the use of any colouring) to show the cell plant, and then the use of  Elodea leaf, to see chloroplasts and the cyclosis. In a foot note we can appreciate the suggestion of "this wonderful material" by Mrs Elsie M. Kupfer, Head of the Department of Biology of the Wadleigh High School, New York City, what suffices alone to show that even at that time, 31 years after Bastin, the use of both materials was of little or no use. In 1922, it is recommended, in just 3 lines, the use of the onion skin in the book of A.G. Tansley, Elements of Plant biology, perhaps indicating a tendency to generalize their use in Anglophone schools.

 While not having other elements of proof, I think, based on the limited selection of materials that I could investigate, that although surely it was used in laboratories from some time before, it is 1890 the first undoubted reference I have at my fingertips on the use of the onion skin, which makes it today, after 120 years:


A Classic Subject for Study

 ·  Onion cells are among the most common choices for cell studies in early biology classes. Easily obtained, inexpensive, they offer samples with no difficult technique required. The thin layer of skin found on the inside of an onion scale (one layer of onion) lifts off without effort and can be wet mounted on a slide with no need for extreme skill. Likewise the cells are large, regular, easily seen and conform very well to the standard generic elements of all plant cells.


A DIC image of the onion epidermis, by Steve Durr, was included in my “Homage to the onion skin” you can see it at this link:


 There is no doubt the stamen hairs of Tradescantia, Zebrina, and perhaps Rhoeo, (which fortunately are at my disposal in Cancun, and which recent taxonomic trends tend to unite all three together in an unique and extensive genus Tradescantia) are great materials.


Bracteas Rhoeo

This is a picture of the floral cradles of Tradescantia (Rhoeo) spatifolia- In the interior of the protective bracteae the little flowers reach maturity one after the other.

Rhoeo, Flor

One mature flower of Tradescantia (Rhoeo), showing the stamens, with its famous staminal hairs. The flower is only 5 mm wide. Canon Powershot A300


Not for nothing do I record their use since the mid-18th century, one hundred and thirty-five years ago. The staminal hairs not only display all the structures of a typical plant cell, but additionally allows an easy exploration of the cyclosis which my onion cells show with difficulty, and even permit to watch mitosis in live, as these photos with a DIC microscope wonderfully display.




But "the tradescantias" are probably much more difficult to obtain and maintain, and even to use (the stamen hairs are very small items, and need to be picked from not fully developed minute flowers), especially for its use in schools at a worldwide range, compared to the humble, large, easy to manipulate and store, and always accessible onion.




  Comments to the author, Walter  Dioni , are welcomed.

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