American Scientist

In his 1767 dissertation, Johannes Roos, a student of the Swedish naturalist Carolus Linnaeus, likened the natural world to a vast, multivaulted museum. The largest rooms were public, open to all, but innumerable tiny rooms were locked, accessible only to specialists with the skill, and luck, to discover the keys. The microscope was Roos’s key to the smallest museum rooms—the mundus invisibilis (invisible world).

Recording the locked room revealed by microscopy is different from merely looking through a lens and being amazed by what is revealed. If general conclusions were to be drawn, ways of looking had to be discovered that ensured observations were specific to neither instrument nor observer—that is, that they were repeatable and not merely artifacts. Moreover, the world revealed by the lens had to be communicated, and therefore depicted with accuracy by natural-history illustrators. Botanical illustrators collaborated with naturalists to present the new worlds made visible through magnification using simple hand lenses or compound microscopes. As tools became more sophisticated and revealed ever more detail, illustrations continued to increase our understanding of plant biology and our appreciation of the aesthetics of the “invisible.”

Drawing Microscopic Objects

The Victorian physician Lionel Smith Beale, professor of physiology at King’s College, London, made a forthright case for the value of illustrating microscopic discoveries in his book How to Work with the Microscope (1868, fourth edition):

It may be truly said that no real advance in our knowledge of the minute structure of animal or vegetable tissues, can be communicated to others, unless accurate drawings are made, for it is almost hopeless for an observer to attempt to describe what he sees in words. . . . On the other hand, a truthful drawing . . . may be compared with drawings which may be made a hundred years hence, and although the means of observation will be far more perfect than they are at present, such comparisons may be useful in many ways, and especially in preventing erroneous conclusions.

When translating images from microscopy to the page, botanical illustrators are faced with three challenges. First, they must get the best performance out of the technology and understand its limitations. Second, they must determine what is presented to the aided eye. Finally, they must decide how to present the observed image in a form that can be understood by the intended audience.

Performance is influenced by the quality of microscope lenses, the ability to focus light, and the methods used to prepare objects for investigation. Some objects can be viewed whole; others must be cut into very thin slices, perhaps stained, then mounted on glass slides before they can be examined and drawn. Preparation, such as preservation and staining, distorts what is observed. Moreover, only a small part of the object is visible under the microscope, making it necessary to refocus the image several times to gain a complete impression, and forcing the observer to stitch together separate images. For example, with the naked eye, a nettle leaf appears to have a few stiff, upright hairs. Under a low-power magnifying glass, the hairs become a tangle. As magnification increases, the structure and character of individual hairs start to resolve.

Early illustrators of microscopic objects were pioneers, exploring without the benefit of frames of reference, comparative illustrations, or even the language to describe what they saw. It is little wonder that viewers of magnified botanical illustrations might disbelieve the evidence of their own eyes, perhaps questioning whether lenses were revealing artifacts of object preparation or monsters of illustrators’ imaginations.

Invisible Worlds

The world revealed in a drop of pond water trapped between two pieces of glass is easy to explore because no complex preparations are necessary. In 1861 the London-based optician and natural-history dealer Andrew Pritchard published the fourth edition of his History of Infusoria, a paean to the microscopic life teeming in this “invisible” aquatic world. Hundreds of “wheel animalcules” (rotifers), protists, single-celled algae, and diatoms were crammed into 40 monochrome and hand-colored metal-etched plates. Such plates both reveal the diversity of microscopic life in water and, together with more than 1,000 pages of text, provide the opportunity to compare individual species.

Diatoms are unicellular organisms, traditionally treated as plants, which are found anywhere there is water on the planet. They are tiny (typically 0.002–0.2 millimeters long), elaborately shaped pillboxes with walls primarily made of glass (silica). To see diatoms at their best, they must be cleaned by being burned or boiled in strong acid to destroy their insides. Their glass carcasses, called frustules, are then mounted in resins so that the silica walls become easily visible under the microscope. The effects are dramatic, which led to diatoms becoming a standard item of interest to Victorian microscopists (see illustration above). Moreover, it was discovered that the sizes of different diatom species and the separations of lines and punctures on frustule surfaces within species were remarkably constant. These consistent sizes led microscopists to include diatoms in other preparations as a ready scale by which to measure the objects being illustrated. Diatoms provided confidence in both the comparability of illustrations and the scientific results being reported.

The 19th-century German zoologist and natural-history illustrator Ernst Haeckel also captured many microscopic aquatic life-forms, such as algae (see illustration above). As a strong advocate of social Darwinism and because of the persistent claims of scientific fraud associated with the embryological images with which he illustrated his research, Haeckel has become an uncomfortable figure in modern biology. But between 1899 and 1904, 100 of his illustrations, lithographed by Adolph Giltsch, were published as Kunstformen der Natur (Art Forms in Nature). These illustrations of microscopic life soon escaped from under the objective lens to become aesthetically powerful images that permeate popular culture today.

The Internal Anatomy of Plants

Stereotypically, plants are sticks rooted to the spot, adorned with leaves and ephemeral reproductive structures. Simplistically, plant cells are like boxes, made from different densities of cardboard, containing water-filled balloons. The denser the cardboard or the more water in the balloon, the more rigid the box. Reduce the amount of water in the balloon and the flexibility of the box increases; that is, the plant begins to wilt, deforming its shape.

English physician Nehemiah Grew was fascinated by plants. He thought that by understanding plants, he might also gain understanding of animals, since, as he wrote in his 1682 book, The Anatomy of Plants, they both “came at first out of the same Hand, and were therefore the Contrivances of the same Wisdom.” The key to Grew’s investigations was the microscope. The Anatomy of Plants included 83 fine copperplate engravings of his own illustrations showing what he saw through a magnifying glass and a microscope.

The anatomical sections presented by Grew are not portraits; they are too perfect, too symmetrical, too uniform. Rather, they are ideals—diagrammatic summaries—the product of hours of observations made while trying to interpret what a pool of light and a pair of lenses revealed to him. Grew described his method of displaying information:

In the Plates, for the clearer conception of the Part described, I have represented it, generally, as entire, as its being magnified to some good degree, would bear. So, for instance, not the Barque, Wood, or Pith of a Root or Tree, by it self; but at least, some portion of all three together: Whereby, both their Texture, and also their Relation one to another, and the Fabrick of the whole, may be observed at one View. Yet have I not every where magnify’d the Part to the same degree; but more or less, as was necessary to represent what is spoken of it.

Grew was experimenting with how to present information from a scale with which most of his audience was unacquainted. The presentation and interpretation of scale remain challenging in plant sciences to the present day.

Grew’s stylized images of magnified transverse sections through tree branches showed how the tissues were arranged and how that arrangement varied among species, revealing a new world of detail and patterns (see illustration above).

By the 19th century, academic journals were replete with exquisite illustrations of sections through plant parts of different species, revealing such subtleties as the thickening of cell walls. These illustrations became essential for researchers to communicate their discoveries to their peers and to convince naturalists about the veracity of their observations. The French plant cytologist and anatomist Charles-François Brisseau de Mirbel was able to show that not only were all plant cells surrounded by a wall, but they all also had a membrane just inside that wall (see illustration below).

Pollen grains offered 19th-century microscopists and botanical illustrators the opportunity to investigate easily prepared, visually attractive objects. By the time the Irish botanist Michael Pakenham Edgeworth produced Pollen (1877), illustrated with 438 figures drawn by Edgeworth and lithographed by the French artist Philibert Charles Berjeau, publishers could rapidly produce highly illustrated botanical works (see illustration below).

Until the mid-20th century, the magnifying glass and the microscope were the tools that enabled botanists to explore the inside of plants and fungi, and the fundamental building blocks that contribute to the appearance of these organisms. Magnification forces an illustrator to become more than an eye, to become an explicit interpreter and editor. They must filter the real from the artifact if they are to depict their subject with accuracy and truth, but without becoming carried away by wonder.

This article is excerpted and adapted with permission from The Beauty of the Flower: The Art and Science of Botanical Illustration by Stephen A. Harris, published by Reaktion Books Ltd, © 2023 by Stephen A. Harris.