Photo: Botanical teaching diagrams created by John Stevens Henslow (1796-1861) and his son George Henslow (1835-1925). These diagrams, numbering over 100, were used to illustrate various plant structures and classifications for teaching purposes.

10th October 2025 marks 200 years since the appointment of CPS co-founder John Stevens Henslow (1796- 1861) as the fourth Professor of Botany at the University of Cambridge in 1825, when Henslow took an oath before the Vice Chancellor as the 'King's Reader in Botany.

Henslow came to Cambridge as an undergraduate student at St John’s College in 1814 and was mainly devoted to mathematics; there were no degrees specialising in the sciences and attendance at any lectures in these subjects was entirely voluntary. Although Henslow studied mathematics, his passion was for natural history, sharing his enthusiasm and wide knowledge of different aspects of natural history to members of the University.

As a young graduate, Henslow would accompany the newly-appointed Woodwardian Professor of Geology Adam Sedgwick on fieldwork. It was on one such geological expedition to the Isle of Wight in the spring of 1819 that he conceived the idea of founding a society to promote research. The Cambridge Philosophical Society was founded shortly afterward along with Adam Sedgwick and Edward Clarke.

Photo: Geological map of Anglesey from J.S. Henslow's 1822 article Geological Description of Anglesea. Article from the Transactions of the Cambridge Philosophical Society Volume 1 pp 359-452.

In 1823 he became Professor of Mineralogy at the University. Botany was however his main interest, and in 1825 he was successful in being elected as Professor of Botany, following the death of Thomas Martyn who had held the Chair for 60 years; he resigned his Mineralogy Chair soon after. His teaching at the time was considered by some as revolutionary by looking at things in a systematic way, pioneering research into the idea of variation in plant species as he wanted to understand the variations, ranges and boundaries of plant species. He introduced practical classes in the Botanic Garden and local countryside, making botany more accessible and engaging.

He drew up in 1829 A Catalogne of British Plants, arranged according to the Natural System, with the Synonyms of De Candolle, Smith and Lindley (12mo), and italicized all plants not found in the county. In a second edition he improved upon this by adding the synonyms of Hooker (Sir Joseph Dalton Hooker), and denoting Cambridgeshire species by the letter c. Another publication On the Examination of a Hybrid Digitalis, 1831 demonstrates Henslow pioneering approach to the use of botanical illustrations, which are best seen in his range of colourful botanical wall charts and diagrams that number over 100, and were used for teaching his students.

At this time the original Botanic Gardens were in centre of the city (now the New Museums Site) where plants were cultivated for medical studies. In 1831 he convinced the University to move its Botanic Garden from the cramped city centre location to the current 40-acre site in 1846. The University herbarium began when the second Professor of Botany John Martyn (1699-1768) gave his Hortus Siccus (literally ”Dry Garden”) and botanical library to the University of Cambridge. His son, the third Professor of Botany Thomas Martyn (1735-1825), meticulously curated his father’s specimens, labelling them with their binomial (genus and species) names, with reference to Carl Linnaeus’s Species Plantarum. Thomas was unable to convince the University to better accommodate the specimens and the herbarium began to deteriorate.

As the forth Professor of Botany, Henslow took the herbarium under his wing and painstakingly remounted over 3,000 of John Martyn’s original specimens, added over 3,500 of his own, and acquired many further specimens through his extensive connections within the British scientific community. Today, the herbarium is housed within the Sainsbury Laboratory, adjacent to Cambridge University Botanic Garden. Outside the entrance to the herbarium is a stone bust of Henslow.

Henslow is most remembered as a friend and mentor of Charles Darwin (1809-1882), a student at Christ's College, for inspiring him with a passion for natural history, proposing him to sail on the HMS Beagle as the naturalist on its five-year voyage, and promoting Darwin’s work as he developed his theory of evolution. Through the Cambridge Philosophical Society, Henslow published the Beagle Letters: Extracts From Letters Addressed to Professor Henslow by C. Darwin, Esq. in 1835.

Photo: Page from the first meeting of the Cambridge Philosophical Society on 30th Oct, 1819 and a page from Extracts From Letters Addressed to Professor Henslow by C. Darwin, Esq, published by Henslow in 1835

The University herbarium is home to around 1,400 botanical specimens that Charles Darwin collected during his voyage aboard HMS Beagle (1831-1835). The Herbarium also holds material collected by Darwin in the UK, including the earliest specimens attributable to him.

Charles Darwin's third son Francis Darwin was President of the Society from 1896-1898 and became a Reader in Botany, spearheading the transition of British plant sciences from a purely systematic study, to one that encompassed plant physiology.

In 2010 the Cambridge Philosophical Society launched its Henslow Fellowships in the fields of Natural Science, Engineering, Mathematics, Computer Science and Clinical Sciences, in honor of one if its founders.

Henslow died at Hitcham in 1861 aged 65 and is buried in the churchyard there; his portrait hangs in the church. Following his death, Charles Darwin wrote of him: “I believe a better man never walked this earth”.

Photo: The Cambridge University Botanic Garden main avenue, with trees planted by Henslow in groups to demonstrate variation within species, species and their hybrids, and mutations – or ‘monstrosities’, as Henslow referred to them.