Darwin’s Botanists: John Stevens Henslow

Specimens of Phleum arenarium in the John Henslow Herbarium, Cambridge University.

John Stevens Henslow is perhaps best known today as Charles Darwin’s botany teacher at Cambridge University and as the person who presented Darwin with an invitation to join the Beagle surveying expedition headed by Captain Robert FitzRoy.  However, this leaves out the herbarium part of the story, obviously the good part.  It’s not surprising that Henslow had an herbarium, but the way he handled his specimens was a little different.  In an article called “What Henslow Taught Darwin,” a team of researchers found that two-thirds of Henslow’s sheets at the Cambridge University Herbarium are what he termed “collated,” that is, having more than one specimen (Kohn et al., 2005).  This doesn’t seem noteworthy, especially for small plants, but their placement is interesting.  They were sometimes arranged in order of increasing height (see above), or with the largest specimen in the middle and specimens in descending height order on either side.  In many cases, specimens on a sheet included ones with different collectors, dates, and locations. 

Many of the specimens were collected around the time Henslow was teaching Darwin and also working on variation within plant species.  He grew Primulas, varying the amount of moisture, manure, and shade, noting that they differed in ways often seen in the field.  Earlier, he had made drawings of the different lengths of styles and stamens in cowslips, Primula veris, something Darwin later investigated.  Kohn and his coauthors argue that the specimens and observations on variation that Henslow presented in his lectures became part of Darwin’s “mental architecture,” so much a part of his thinking that he might not have even realized the debt he owed to Henslow.  This wasn’t just from the three cycles of Henslow lectures that Darwin attended.  Janet Browne (1995) notes that they became closest during Darwin’s last months at Cambridge when they often went on walks and collecting trips, and dined together at Henslow’s house.  It was then that Henslow arranged for his student to accompany Adam Sedgewick, Cambridge’s professor of geology, on a field trip to Wales.  This cemented Darwin’s interest in geology, which was important to his observations on the Beagle.  It was also when Darwin’s oldest known herbarium specimens were collected, three Matthiola sinuata, that were collated by Henslow on a single sheet along with another example collected by a Miss Blake.    

Darwin had an opportunity to collect many plants on the Beagle expedition, hundreds of them, which he sent to Henslow along with animal skins, fossils, etc.  After shipping the first package, he had to wait two years for a letter from Henslow to catch up with him.  In the meantime, he worried that no correspondence meant that Henslow wasn’t pleased with what he sent.  He was relieved to finally read that Henslow was grateful for the materials.  However, his mentor did comment that Darwin shouldn’t send scraps, that the entire plant should be included when possible—leaves, roots, stem, flowers—and that one of the leaves should be turned back to reveal the underside.  Also, it wasn’t necessary to sew down the specimens; they traveled better when left loose (Allan, 1977). 

Soon after his return to England, Darwin delivered the last batch of specimens in person, and Henslow agreed to begin work on identifying them.  However, this turned out to be a slow process, in part because the assignment coincided with Henslow’s move from Cambridge to become vicar at a church in Hitcham, almost 50 miles away.  He remained a Cambridge professor, but usually only visited there to give his lectures.  Another problem was that so many of the plants were unfamiliar to Henslow.  Darwin kept prodding him for several years, until eventually Henslow turned the specimens over to the young botanist Joseph Dalton Hooker, who had himself just returned from a round-the-world expedition with the British Navy.  Hooker found that many of the plants that Darwin collected on the Galapagos Islands were endemic to the islands, and in many cases, occurred on only one Island, information that Darwin was relieved to hear since it fit with his observations on birds and other animals.

Henslow might not have had time for Darwin’s plants, but he made a number of other contributions to botany (Walters & Stowe, 2001).  He revitalized the botany program at Cambridge University.  It had become rather dormant under the nearly 60-year reign of Thomas Martyn, who spent much of his time as a London physician.  He overhauled the herbarium, which had become disorganized.  Henslow made it known that he was building the collection, particularly with plants from Cambridgeshire and Great Britain, and received notable donations.  He pressed for a new, larger botanic garden at the university to replace the one that had fallen into disrepair and then oversaw this project.  Henslow’s lectures were well-received, and they were accompanied with specimens and with charts he drew.  He wrote and illustrated a text to go with his course, and later, with help from his daughter and the noted artist and engraver Fitch, published a series of large botanical charts for sale to schools (Burk, 2005).  Henslow wrote papers on plant variation and on zoological aspects of natural history, most before his move away from Cambridge.  In all, he produced significant contributions to botanical science besides serving as Darwin’s mentor and travel arranger. 

References

Allan, M. (1977). Darwin and His Flowers: The Key to Natural Selection. New York, NY: Taplinger.

Browne, J. (1995). Charles Darwin: Voyaging. Princeton, NJ: Princeton University Press.

Burk, W. (2005). Henslow’s wall charts: A legacy of botanical instruction. Bulletin of the Hunt Institute for Botanical Documentation, 17(1), 4–6.

Kohn, D., Murrell, G., Parker, J., & Whitehorn, M. (2005). What Henslow taught Darwin. Nature, 436, 643–645.

Walters, S. M., & Stow, E. A. (2001). Darwin’s Mentor: John Stevens Henslow, 1796-1861. Cambridge, UK: Cambridge University Press.

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