Through his own collections, the herbarium was developed by Henslow in Cambridgeshire while being on holiday or at home with his family. A network of collaborators across the country also made significant contributions in his collection along with Leonard Jeyns – his friend from Cambridge.
Due to his meticulous annotation of sheets, the growth of Henslow’s herbarium got created. In a set order, the name of the collector, date of collection, place of collection, and species was recorded by Henslow. Numbers were also added to plants for their identification since they were obtained from various places. Further for defining the species, disparate plant collections were also combined by Henslow. Moreover, branching patterns, leaf shape, and variation in size were shown by collated sheets. Such kind of collation could never be carried out at that specific time period by any of the leading botanists.
A list indeed did was published, known, as ‘A Catalogue of British Plants’ in 1829. This list was created as per the natural system. Later, this list was also used by Darwin in 1829, 1830, and 1831 whenever he attended the five-week course of Henslow in the summer term.
Variation in plant size was shown mostly by the herbarium sheets of Henslow. He also made extensive usage of terminology ‘monstrosity’ referring to the sudden changes of form. This terminology is now used to refer to major developmental abnormalities and mutations. Moreover, papers based on monstrosities in Acer, Reseda, and Adoxa were also published by Henslow. The key objective of these papers was to address the laws that govern nature. Along with this other insights were also gained by the study of hybrids. A sterile spontaneous hybrid Digitalis along with its two parents was also analyzed by Henslow. It was proposed by Henslow that the laws of heredity can be revealed by a hundred of various hybrids.
To great effect, the artistic ability was used by Henslow. To illustrate his lectures once, around seventy elephant-folio drawings were prepared by one of the new professors he assigned the responsibility. In 1828, the lecture of Henslow was appreciated with enthusiasm due to the excellence level exhibited in these illustrations. Charles Darwin was also suggested by his cousin William Darwin Fox not to miss these lectures. Around three times, the course of Henslow was taken by Charles. These were the only lectures that Charles attended with great interest at Cambridge. In Cambridge, a few years ago some of these illustrations were explored comprised of around folios of 45 artists.
One of the main plants which were central for the understanding of Henslow about species and varieties was the genus Primula. Hybrids known as false oxlips as well as three major species, such as P. elatior (oxlip), P. veris (cowslip), and P. Vulgaris (primrose) were found around Cambridge. These were considered as single species with three varieties by Henslow. Varieties of a single species were suggested by some plants that were both cowslip/oxlip-like (with umbels of flowers) and primrose-like (single flowers on long stems). In the cultivation experiments, the changes were noted from one to the other.
Henslow invested a lot of years in studying Primula. The Primula experiments of Henslow was repeated by Darwin even twenty years late for collecting botanical evidence for the instability of species instead of stability.