Will the real Robert Hooke please stand up?

Dear Editor,

Englishman Robert Hooke (1635‒1703) is commonly credited with the introduction of the term cell into biology following his observations of cells in cork, a tissue in the bark of some woody plants (Reynolds 2018). A recent description of his discovery in this journal reads: “In 1665, Robert Hooke peered through his primitive microscope at a slice of cork and described little boxes he called ‘cellula’—rooms that monks inhabited. These ‘cellula’ were dead cells, and all that remained visible to him were their cell walls” (Delmer et al. 2024, p. 1257; for similar statements, see Cheung et al. 2022, p. 53, and Gu and Rasmussen 2022, p. 103). No supporting reference was provided in any of these cases, assumedly because this story of Hooke's discovery is considered common knowledge—after all, it is told in textbooks (e.g. Levetin and McMahon 2008, p. 20; Lodish et al. 2016, p. 129; Johnson 2020, pp. 28, 66) and on authoritative educational websites (e.g. Alberts 2024). Yet it is just a story. No monks or monasteries are found anywhere in Hooke's writings on cells, and neither is the Latin cellula (a diminutive of cella). The reiterations of this “monastic myth” in The Plant Cell of all journals call for a clarifying comment.

Serving as the Curator of Experiments of the Royal Society of London soon after it was founded in 1660 (Jardine 2005), Robert Hooke used a compound microscope to investigate the previously inaccessible fine structure of various materials (Lawson 2016). Rather than merely “peering at a slice of cork,” Hooke developed indirect illumination techniques and studied sections cut in various planes to reconstruct the 3-dimensional structure of various plant materials including cork (Fig. 1A; Hooke 1665, pp. 112‒121). As he explained in Micrographia: or some physiological descriptions of minute bodies made by magnifying glasses with observations and inquiries thereupon, he did not consider cork a part of the plant, but “something analogus to the Mushrome, or Moss on other Trees” (Hooke 1665, p. 115). He stressed, though, that he had found the same “texture” as in cork in the pith of numerous plants, listing eight species by name (Hooke 1665, p. 115). It is incorrect that Hooke only saw dead cell walls (Matzke 1943); in fact, he noted that cells in plants were “fill’d with juices” (Hooke 1665, p. 116). This was an essential observation in his contemporary theoretical context: according to the fiber doctrine, living matter generally consisted of networks of fibrous materials that directed and regulated the flow of fluids within the living body (Berg 1942; Ishizuka 2012; Baldassarri 2024; Peters 2024a). Consequently, Hooke interpreted linear series of cells as narrow pipes that were subdivided by “valves, or diaphragms” to regulate fluid flow within the pipes (Fig. 1B). While Hooke failed to visualize any openings in these valves—which he ascribed to the insufficient power of his microscope—he clearly observed how fluid-filled cells in freshly prepared samples were “sweating out” their contents when the specimens dried. Evidently, the intracellular fluids could permeate the cell walls as required by his interpretation (Hooke 1665, p. 116). In an earlier chapter, he had reported analogous observations in wood without using the term cell (Hooke 1665, p. 107).

Figure 1.

Robert Hooke's observations A) and reconstructed interpretation B) of the microscopic structure of cork. A) Hooke reported that cork sections taken perpendicular to the stem surface exhibited rows of more or less rectangular cavities (left), whereas irregularly shaped and arranged cavities showed in sections cut parallel to the stem surface (right; Hooke 1665, Fig. of Scheme XI; source: https://digital.sciencehistory.org/works/9g54xj51s). B) Hooke concluded that he saw co-aligned pipes, which he called “pores” (blue), oriented from the center to the periphery of the stem. Movements of nutritive fluids in these pipes were regulated by “valves, or diaphragms” that traversed the pipes at regular intervals (yellow). Hooke called a pipe segment between two consecutive valves a “cell.”

Subdivisions of pipe-like structures, in which, according to contemporary biological thought, regulated transport and modifications of important materials occurred, had been called cellae or cells long before Hooke, including the cells of the colon, uterus, seminal vesicle, and brain (Peters 2024b). The functional analogy between these widely known macroscopic cells and the microscopic structures Hooke described in plants probably was obvious to his educated readers, even though he did not explicitly state it in Micrographia (Peters 2024b). Hooke chose to use an analogy between the cells in cork and those in honeycomb, which appeared similar to him due to structural features such as the ratio of wall thickness to cell size (Hooke 1665, p. 113).

Hooke's inferences concerning cell function were by no means idiosyncratic. Scholars like Nehemiah Grew and Antonie van Leeuwenhoek, who studied plant structure in greater detail, generally interpreted plant cells as elements of coherent pathways for regulated fluid flows (Grew 1673, pp. 107–110; van Leeuwenhoek 1705; for discussion, see Peters 2024a). This implied, of course, that cell walls possessed an adjustable permeability for the flowing fluids. The general interpretation changed in the middle of the 19th century when, following the work of Matthias Schleiden and Theodor Schwann, many European biologists began to view cells as elementary organisms, structurally isolated and functionally autonomous biological atoms that assembled to form multicellular composites (Nicholson 2010; Reynolds 2018). Hooke's work soon became part of the founding myth of the new cell theory. The notion of the cell as a structurally isolated entity was projected onto historical concepts in general, while the polysemous word cell was arbitrarily pruned down to a single meaning. This resulted in the assumption that by cell, older biologists could not possibly have meant anything but a closed chamber (Peters 2024a). The historical and semantic distortion is manifest in statements by late 19th-century scholars who rejected the (by now dogmatic) cell theory: “the term ‘cell’ is a biological misnomer; for whatever the living cell is, it is not, as the word implies, a hollow chamber surrounded by solid walls” (Wilson 1896, p. 13). Leading botanists concluded that Robert Hooke had coined the term “due to an error” (Sachs 1892, p. 60), taking Hooke to task for the opposite of what he actually had described.

Analogies and metaphors have important functions in colloquial speech (Lakoff and Johnson 1980) as well as in scientific discourses; “metaphors do make a real difference to how science is done” (Reynolds 2018, p. 3; see also Keller 2002; Nicholson 2019; Kampourakis 2020; Reynolds 2022). Since connotations of a metaphor's source transfer to its target, polysemy may create ambiguity. My claim that something “is like a cell” will be understood differently depending on whether I imply storm cells, prison cells, unit cells, electrical cells, fuel cells, terrorist cells, or cells we make calls with. Consequently, replacing honeycombs with monasteries as the source of Hooke's cell metaphor affects our understanding of what Hooke said about plant cells. Honeycomb cells provide essential infrastructure for the industrious interactions of bees with their environment (food production and storage) and for reproduction (brood chambers for larvae). In contrast, monks in Christian monasteries live more or less isolated from the outside world, and the monk's cell carries connotations of seclusion, insularity, and solitude. The alleged analogy between Hooke's plant cells and monks' cells aligns well with the view of biological cells as structurally isolated, autonomous units. The apparent credibility of the myth among biologists thus may be explained by the congruence of the connotations of the monk's cell and central tenets of the classical biological cell theory. But for Hooke, plant cells were elements of the “pipes through which the succus nutritius, or natural juices of Vegetables are convey’d” (Hooke 1665, p. 114). The plant cell type recognized today that most clearly exhibits the functional features that Hooke ascribed to what he called cells is the sieve element (Knoblauch and Peters 2013).

Robert Hooke's first application of the word cell to structures we still call by that name certainly can be an inspirational point—but rather than perpetuating an easily disproved founding myth to make that point, why not use the original source? Like older scientific work in general, Micrographia is in the public domain and available electronically for free use in the classroom (see References). In my courses, inexperienced students found it easier to cut thin sections in various planes in cork than in more delicate live plant organs, enabling them to reproduce Hooke's analysis of the tissue's 3-dimensional structure (for a reconstruction of what Hooke actually saw in cork, see Fig. 5 in Peters 2024a). The symplasmic organization of plant tissues (Peters et al. 2021) and the mechanisms of symplasmic transport (Peters and Knoblauch 2022) are notoriously difficult topics, and I suspect that our students' perspective will improve if we replace the monastic myth by a realistic summary of Hooke's interpretation in textbooks and classrooms. Finally, the comparison of Hooke's conclusions with modern concepts demonstrates that interpretations of empirical findings depend on theoretical contexts, an insight that may foster a deeper awareness of the complexities of the scientific endeavor.

It's about time—let us ask the real Robert Hooke to stand up and put the monastic myth to rest!

Data availability

There are no new data associated with this article.