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Writer's pictureSentient Forest Project

The Scent Intelligence of Forests by John C. Ryan

This is an excerpt from a longer essay on Henry David Thoreau's engagement with plant olfaction.


Fragrance is a marker of botanical intelligence. Produced by all parts of the plant, volatile organic compounds—or VOCs—are chemical cues facilitating communication among plants as well as between plants, other life forms, and the abiotic domain. Responsible for distinctive scents and thus known as odorants, these compounds become smells when detected by biological sensors—nasal or otherwise. The recognition of VOC signatures by enzymes in plants constitutes a “sensory capability akin to smell in animals.”[3] The “massive diversity of VOCs” includes more than 1,700 known compounds.[4] VOCs mediate plant-to-plant signaling, promote plant-pollinator exchanges, and enhance seed dispersal.[5] In response to diseases, herbivory, and other environmental factors, plants send and receive information-dense chemical cues. Volatiles released from damaged leaves, for instance, prepare nearby plants to bolster their defenses.[6] Changing with nocturnal and diurnal conditions, VOCs respond in accordance with plants’ “endogenous biological clock.”[7] Thoreau’s cognizance of the intensification of the apple trees’ “volatile and ethereal quality” during the evening reflects awareness of the temporality of plant olfaction.[8] 


Plants are endowed with a chemical language. Although sessile, they communicate intraorganismically (within their bodies), interorganismically (between the same and different species), and metaorganismically (between plants and non-plants).[9] As a “plant language,” volatiles function as signals.[10] The “volatile and ethereal quality” apprehended by Thoreau is a sign of plants in communication with others. Olfaction relays an immense amount of information to plants and their fellow denizens.[11] VOCs are an “airborne fingerprint of the metabolic state of the leaf or plant.”[12] Nearby plants are known to eavesdrop on “olfactory conversations” to acquire knowledge about their habitats.[13] Olfaction is always a dialogue insofar as a plant delivers “a volatile presentation to an audience” and, in turn, receives “chemically encoded information from other sources”[14] While individual VOCs are comparable to words in a lexicon, quantities and proportions of the compounds result in the expression of bouquets, signatures, or “sentences.”[15] That the volatile bouquet varies between species and, even, between individuals signifies the dialectal specificity of vegetal languages.[16] Accordingly, it is possible to reimagine Thoreau’s orchards, gardens, fields, and forests as odorscapes, a term denoting the spatiotemporal distribution of volatiles. These places are the loci of his engagement with plant olfaction.


Thoreau’s narratives uphold the importance of human-plant exchanges within olfaction. By stimulating the nervous and endocrine systems, plant odors benefit human health. Olfactory stimuli in gardens induce physical and mental wellbeing.[17] Exposure to forest volatiles decreases anxiety, improves cognitive performance, and promotes cardiovascular health.[18] In March 1859, Thoreau noted the “affecting” quality of forest scent:  

 

As I look toward the woods (from Wood’s Bridge), I perceive the spring in the softened air. This is to me the most interesting and affecting phenomenon of the season as yet […] There is a fine effluence surrounding the wood—as if the sap had begun to stir—and you could detect it a mile off. (OJT 28, 238, 239)

 

Perceiving spring’s emergence through the “fine effluence” of sap offered Thoreau a mode of communion with arboreal nature. That the sylvan aroma is detectible “a mile off” signifies the capacity of odor to transcend demarcations between bodies, senses, and places. His reveling in the season’s scent affirms how olfactory experience is “profoundly immersive” and “deeply transformative.”[19] Thoreau observed how trees “put out feelers—by which the senses apprehend them more tenderly” (OJT 28, 239). A plant neurobiological perspective suggests that these “feelers” were forest volatiles, some of which are more active during the spring.[20] Thoreau’s detection of the effluence of sap underscores the power of smell to catalyze communication between people and plants at a distance.  

 

The conjunction of scent, communication, plant intelligence, and human perception informs Thoreau’s narrative of goldenrod, a species known for its licorice-scented leaves. Whereas the genus derives from the Latin solidus for “whole” and ago for “to make,” the species epithet odora denotes “fragrant.”[21] The etymology suggests that, inhaling the plant’s fragrance, one can be made whole—one can be healed. Thoreau’s account reveals his familiarity with Jacob Bigelow’s American Medical Botany.[22] Bigelow noted the goldenrod’s “transparent cells, which constitute the dotting of the leaves.”[23] In August 1853, Thoreau likewise witnessed “leaves full of pellucid dots and yielding, after being in my pocket all day, a very pleasant fragrance” (J 5, 353). Several days later, he encountered goldenrod’s inimitable scent:     

 

I find the Solidago odora out by the path to foot of cliffs beyond Hayden’s, maybe twenty or thirty rods into woods about the summit level. It is said to have the odor of anise. It is somewhat like that of sassafras bark. It must be somewhat dried and then bruised. (J 5, 361-62).

 

The passage echoes Bigelow’s characterization of goldenrod as effusing “a delightfully fragrant odour, partaking of that of anise and sassafras, but different from either.”[24] For Thoreau, a sense of indeterminacy persists: the sweet goldenrod “is said to have the odor of anise” and “is somewhat like that of sassafras bark.” Intimated by both writers, the ineffable difference of goldenrod’s fragrance highlights the insufficiency of the English lexicon to evoke smell.

A feature of Thoreau’s engagement with plant olfaction, then, is his search for a vocabulary to summon scent in all its variation. According to neuroscientists, olfactory naming is inherently challenging and “elusive” due to the limitations of the odor-language neural system.[25] Thoreau mitigates the inadequacies of language by embedding scent encounters within narratives of tactile interaction. For its scent to emanate, dry goldenrod must be bruised, a process expedited by keeping leaves “in my pocket all day” (J 5, 353).


Participation in plant olfaction recurs in reference to aromatic pennyroyal. Signalled by the repetition of the verb bruise, human action accelerates plant volatile emission. In August 1850, he perceived “the pennyroyal which my feet have bruised” and, three months later, delighted in the smell of “the pennyroyal which I had bruised, though this dried up long ago” (J 2, 104). Correspondingly, in August 1853, he “smelled pennyroyal, but it was only after a considerable search that I discovered a single minute plant, which I had trodden on” (J 5, 358). The fortuitous bruising of leaves agitated plant volatiles, prompting human-flora relations. In November 1857, he exclaimed, “How often I have found penny royal by the fragrance it emitted when bruised by my feet!” (OJT 24, 593). Physical intervention energizes the scent intelligence of plants. In Thoreau’s view, scent is integral to communication yet also closely associated with memory.


[1] Jonik, “‘Wild Thinking’,” 97, 101.

[2] Henry David Thoreau, [1862] Wild Apples (Boston: Houghton, Mifflin and Company, 1923), 12.

[3] Trewavas, “The Foundations,” 14.

[4] Jesús Picazo-Aragonés, Anass Terrab, and Francisco Balao, “Plant Volatile Organic Compounds Evolution: Transcriptional Regulation, Epigenetics and Polyploidy,” International Journal of Molecular Sciences 21, no. 23 (2020): 1–18, 1, 10.

[5] Picazo-Aragonés et al., “Plant Volatile,” 1–2.

[6] Nagashima et al., “Transcriptional Regulators,” 2256.

[7] Salma Mostafa, Yun Wang, Wen Zeng, and Biao Jin, “Floral Scents and Fruit Aromas: Functions, Compositions, Biosynthesis, and Regulation,” Frontiers in Plant Science 13, no. 13 (2022): 1–23, 10.

[8] Thoreau, Wild Apples, 12.

[9] Guenther Witzany, “Plant Communication from Biosemiotic Perspective,” Plant Signaling & Behavior 1, no. 4 (2006): 169–78, 170.

[10] Trewavas, “The Foundations,” 13–14.

[11] Daniel Chamovitz, What a Plant Knows: A Field Guide to the Senses (New York: Farrar, Straus and Giroux, 2012), 27.

[12] J. Keaton Wilson, André Kessler, and H. Arthur Woods, “Noisy Communication via Airborne Infochemicals,” BioScience 65, no. 7 (2015): 667–77, 667. 

[13] Chamovitz, What a Plant, 43.

[14] Robert Glinwood and James D. Blande, “Preface,” in Decipering Chemical Language of Plant Communication, ed. James D. Blande and Robert Glinwood (Cham, Switzerland: Springer, 2016), v–vi, v.

[15] Jarmo K. Holopainen and James D. Blande, “Molecular Plant Volatile Communication,” in Sensing in Nature, ed. Carlos López-Larrea (New York: Springer Science+Business Media, 2012), 17–31, 17.

[16] Trewavas, “The Foundations,” 14.

[17] Xinguo Zhang, Jiayu Guo, Xiaowan Zhang, and Qixiang Zhang, “Physiological Effects of a Garden Plant Smellscape from the Perspective of Perceptual Interaction,” International Journal of Environmental Research and Public Health 20, no. 5004 (2023): 1–15.

[18] Michele Antonelli, Davide Donelli, Grazia Barbieri, Marco Valussi, Valentina Maggini, and Fabio Firenzuoli, “Forest Volatile Organic Compounds and Their Effects on Human Health,” International Journal of Environmental Research and Public Health 17, no. 18 (2020): 1–36; Francesco Meneguzzo, Lorenzo Albanese, Giorgio Bartolini, and Federica Zabini, “Temporal and Spatial Variability of Volatile Organic Compounds in the Forest Atmosphere,” International Journal of Environmental Research and Public Health 16, no. 24 (2019): 1–24.

[19] Howes, The Sensory, 190.

[20] Meneguzzo et al., “Temporal and Spatial,” 2.

[21] Missouri Botanical Garden, “Solidago odora," Gardening Help, 2024,  www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?taxonid=277477&isprofile=0&n=1

[22] Jacob Bigelow, American Medical Botany, vol 1 (Boston: Cummings and Hilliard, 1817).

[23] Bigelow, American, 189.

[24] Bigelow, American, 189, emphasis added.

[25] Jonas K. Olofsson and Jay A. Gottfried, “The Muted Sense: Neurocognitive Limitations of Olfactory Language,” Trends in Cognitive Sciences 19, no. 6 (2015): 314–21, 314.

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