In my view, you can never have too much of a good thing if that thing involves the Cambridge University Botanic Garden, so I was delighted a few days ago to participate in a study session on J.S. Henslow’s influence on Darwin as a botanist, led by Christine Bartram, the custodian of the Cambridge University Herbarium.

We started with a hands-on session, mounting a pressed plant specimen (in my case sainfoin, Onobrychis viviifolia Scop. subsp. viciifolia: ‘Scop.’ is Giovanni Antonio Scopoli, (1723–88), a physician and naturalist from the Trentino region, called the ‘Linnaeus of the Austrian Empire’). It is a fiddly business, but at least my sainfoin was flat – others were wrestling with hawthorn branches. The object is to display as much of the plant as possible, attaching it to the archive-grade paper with strips of archive-grade sticky paper, and focusing on securing the plant rather than achieving an aesthetic result. As with stiff meringue, one should be able to hold the sheet over one’s head without anything falling off. It took us about 20-30 minutes to achieve security.

My mounted specimen.

In this context it is worth remembering that when Henslow succeeded to the post of professor of botany, he found the herbarium in poor condition, through neglect which had led to storage in conditions which encouraged damp, mould and non-beneficial insects. He set out (single-handed as far as is known) to remount and relabel what was salvageable, at the same time continuing to press and mount his own finds carefully, using good rag paper (Whatman wove, no less, as used by Baskerville and Blake, but that’s for another blog piece). He gave Darwin strict instructions on how to press and label the plant specimens his ex-pupil sent back from the Beagle voyage, and his insistence on careful labelling means that the variant types of Scalesia found on different islands in the Galapagos can be more clearly distinguished than the Darwin finches, whose labelling was famously not up to the mark.

Solanum tuberosum, collected by Darwin on the Chonos archipelago, off the coast of Chile: Henslow has a question mark as to the exact date in December 1834.

Henslow passed all the plant specimens he received but did not recognise to J.D. Hooker in the first instance, for identification and naming; only afterwards did he begin to examine their form and function himself. On this sheet showing Scalesia darwinii (named by ‘Hook.fil.’ in honour of the plant’s collector and his friend), Hooker’s writing can be seen in black ink, above the much more graceful hand of Henslow, which gives the specific Galapagos island, the date collected, and the note that this form is characteristic of James Island (San Salvador). The specimen was later (in 1976) chosen as the holotype for the species.

Scalesia darwinii, from James Island (now San Salvador), with annotations ranging over more than 100 years. The different species of Scalesia come from different islands of the Galapagos archipelago.

Scalesia incisa, from Chatham Island (now San Cristobal).

Scalesia affinis, from Charles Island (now Floreana).

This lack of interest in names per se was a feature of Henslow’s own rather unusual approach to botany. Unlike most of the natural scientists of his time, who were concerned with classification and the naming of plants on the basis of perceived resemblance to other plants, he was not much of a taxonomist. Naming had been THE major activity ever since Linnaeus, regardless of whether you were a follower of Linnaeus, of Jussieu or of De Candolle (whose Prodromus Henslow used to categorise plants, and whose ideas he introduced into Britain): one thinks of the young Joseph Banks on Cook’s first voyage, enthusiastically applying binomial nomenclature to every living thing he came across.

What Henslow wanted was to know why things were as they were, and especially – as a Church of England clergyman and a creationist – what were the boundaries of variation in any species. (All species were of course created by God and were immutable – except that they did seem to vary.) He began to observe and collect ‘monstrosities’, strange, unusual forms of plants, with more or fewer leaves or petals than usual, greater or lesser height, unusual colour or variegation, or any other feature which did not match the ‘normal’ manifestation of the plant.

One of Henslow’s large-scale monstrosities, Fagus sylvatica miltonensis, a grafted beech in the Botanic Garden.

Henslow’s paper on monstrous mignonettes – usually tiny and inoffensive plants.

A dried specimen of common mignonette (Reseda odorata).

He brought to his task terrific powers of observation, unbelievable patience, great skill as an artist, and an analytical approach which probably derived from his original studies as a mathematician. I discovered to my astonishment that Henslow was the first person to record an observation which is now a stalwart of the primary school nature table (assuming they still have such things): the centres of primroses come in two kinds, pins and thrums (or thumbs). Plants of one type can only fertilise the other, as Darwin demonstrated in a paper published a year after Henslow’s death.

Primrose and cowslip flowers, drawn by Henslow.

Henslow himself wrote several papers about the Primula family, as they frequently show variety in colour, as well as flower structure. We saw several of his specimen sheets, some collected locally at Coton and Madingley, the Gog Magog hills and Swaffham Prior, others from Devon and Cheshire.

Primula specimens collected in Cambridgeshire and elsewhere.

He had a network of collectors, some his students or ex-students, others botanists (including his brother-in-law Leonard Jenyns, and Hooker, later to be his son-in-law), of whom he had created a network by appeals such as this, at the end of an article about the variation in numbers of leaves of 1500 examples(!) of Paris quadrifolia which he had collected at a Coton farm.

Two of the many Paris sheets in the Herbarium.

Engraving in Henslow’s article of the parts of Paris quadrifolium.

It is clear that many of the composite pressed sheets Henslow produced were intended to demonstrate variation, either of leaf shape or size: he often placed specimens across the sheet from shortest to tallest, or in a bell curve shape (though there is also a practical reason for this: put too many plants with bulky flowers in the centres of sheets, and you can’t get the press to tighten uniformly, leading to the likelihood that the specimens will not dry out completely).

Small and tall samples of Zinnia paucifolia collected by Darwin at the Cape Verde Islands.

Henslow used his considerable skills to draw and paint posters as teaching and lecturing aids.

As far as is known, Henslow did not accept Darwin’s Dangerous Idea: at any rate, he never committed himself to an opinion in writing. It must have been extremely difficult for him to have been in the chair of the session of the British Association in Oxford in June 1860 during which T.H. Huxley and Bishop Samuel Wilberforce had their famous and forceful exchange of views: Hooker reported to Darwin that ‘Henslow as President would have none speak but those who had arguments to use’, as opposed to those who merely wanted to shout down the other side.

But as Jenyns points out in his biography of his brother-in-law: ‘He declined going a step further than he could see his way … he had told me he thought there were in Darwin’s book too many suppositions, too many things assumed, which might or might not be true.’ Henslow’s way of doing science seems to have been to gather samples, annotate them, preserve them, take the widest possible view of their likeness to or dissimilarity from other samples, and come to tentative inferences on that batch of samples alone: to draw wider conclusions across the whole of nature and through the whole of geological time was to argue ahead of his data – something which he, like Sherlock Holmes, was not prepared to do.

And anyway, what with his writings, his lectures, university politics, his mentoring of students, his cure of souls at Hitcham in Suffolk, and the new Cambridge Botanic Garden (and three herbaria: plants of Cambridgeshire, plants of Great Britain, and the general collection (which contained no fewer than 2,400 specimens contributed from the Beagle voyage alone, all mounted and labelled by him at say, 20 minutes each)) which he was building for the university, Henslow may simply not have had the time to theorise…

Caroline

P.S. I took the opportunity while at the Herbarium to snap this early twentieth-century vasculum, or plant collector’s metal case (as in E.H. Wilson’s memoir). I’d never got around to finding out what a vasculum looked like, and here it was, on a plate, so to speak…