On the death of Dr E. D. Clarke, he offered himself for the Professorship of Mineralogy. Chemistry, as well as the study of Minerals, now occupied his attention. He was only 26 years of age, and still B.A., when elected to that chair. At the age of 27 he published his Syllabus of Mineralogy in 1823, "A useful manual of reference to all persons studying Mineralogy, independently of the immediate circumstances which led to its publication[98]."
In 1827 Prof. Martyn died and Prof. Henslow was elected to the chair of Botany, being succeeded by Whewell on resigning the Professorship of Mineralogy. He now turned his attention to the study of Botany; but he never paid much heed to systematic botany, for his taste lay in the direction of what is now called Ecology. He then wrote "Botanists would rather receive one of our most common weeds from a newly-discovered or newly-explored country, than a new species of an already known genus. There are higher departments of Botany than mere collectors of specimens are aware of; for to ascertain the geographical distribution of a well-known species is a point of vastly superior interest to the mere acquisition of a rare specimen." À propos of this he made elaborate epitomes of the Botanical Geographies of De Candolle, and of the writings of Humboldt, Poiret and others. His MS. is not unlike a fore-runner of Schimper's Botanical Geography of to-day. He thus expressed himself in the Introduction to his Descriptive and Physiological Botany (1836):—in the second section headed Botany ... "This enquiry should extend as well to the investigation of the outward forms [of plant organs] and the conditions in which plants, whether recent or fossil, are met with, as to the examination of the various functions which they perform whilst in the living state and to the laws by which their distribution on the earth's surface is regulated." Again, in the Preface to the Flora of Suffolk by himself and E. S. Skepper, he wrote:—"We had thought of saying something in regard to the Geographic distribution of the species, but found our material insufficient for treating this question to advantage." As an alternative he suggests interleaving the 'Catalogue,' as the book was also called, in which observers could add observations on the Geological formations and superficial soils upon which each species grows, e.g. Chalk, the Crags, Gravels of post-tertiary period, &c. as well as maritime, marshy, boggy, healthy and cultivated soils[99].
Though he wrote against mere collecting, he was an insatiable collector himself; but it was always with some definite, useful and generally educational purpose, and the best of his collections invariably went to museums, especially those of the Philosophical Society of Cambridge, of Kew and of Ipswich. The first still has the fishes he collected at Weymouth in 1832, solely for his brother-in-law L. Jenyns, the author of The British Vertebrate Animals.
One of the first things to which his attention was directed was the Cambridge Botanic Garden. It was far too small and in the centre of the town, where the scientific buildings are now erected. He urged the necessity of a new one, but it was not till 1831 that the present site was secured; the first tree, however, was not planted until 1846.
His educational method of teaching was totally different from the mere instructional method of all previous lecturers. To cram up facts was the students' duty in the Medical schools, where botany was supposed to be taught. To learn by their own discovery was his new method, and so each student educated himself by examining and recording plant structures first seen by his own dissections. Having long been in the habit of observing himself, he was early convinced of the importance of practical work and he always had "demonstrations," as he called them, from living specimens. Each member of the class had a round wooden plate for dissecting upon. He had only sixteen lectures to give, but he succeeded in arousing an enthusiasm in some, and interest in all who attended, and thus many came besides undergraduates, as Dr Ainslie, the Master of Pembroke.
The value of "practical work" put a stop to cram, and he was the first to introduce the examination of flowers, not only at Cambridge but for the degrees in the University of London. "He insisted," wrote Dr Hooker, "that a knowledge of physiological botany, technical terms, minute anatomy, &c. were not subjects by which a candidate's real knowledge could be tested, for the longest memory must win the day, the less did it test the observing or reasoning faculties of the men. He, therefore, insisted in all his examinations that the men should dissect specimens, describe their organs systematically and be prepared to explain their relations, uses and significations in a physiological and classificatory point of view; and thus prove that they had used their eyes, hands and heads, as well as their books[100]."
His natural bent and interest were in the investigations of the phenomena of plant-life, e.g. the colours of flowers, the laws of phyllotaxis and what would now be called biometrical studies, e.g. of the variations in the leaves of Paris and the cotyledons of the sycamore, hybridization, teratology and the origin of varieties, etc. The geographical distribution of plants and the effects of external agencies upon them were also specially studied, as is recorded in the note-book mentioned. He was thus a genuine Ecologist without knowing it. He published about 50 papers on botanical subjects during his professorship from 1825 to 1861, in which he was more than once the pioneer of special branches of study since taken up, as in the above mentioned hybridization and varietal differences under cultivation, etc.; for experiments were made on the specific identity between the Primrose, Oxlip, Cowslip and Polyanthus. He raised many varieties, which were often permanent or "Mutations"; though sometimes reversions appeared, concluding that when one form thus changed to another that was sufficient proof of identity.
Though his occupations were necessarily much changed at Hitcham, of which he became the Rector in 1838, from those at Cambridge, he by no means neglected science; but he utilized it in different ways. Thus having a good knowledge of chemistry, he endeavoured to make the farmers interested in more scientific methods of farming than they had been accustomed to. He gave lectures on the fermentation of manures and he wrote fifteen "Letters to Farmers," first published in the Bury Post and then separately. He even proposed that they should make experiments themselves. For this purpose he issued schedules to about 70 farmers who asked for them.
The experiment was to test Liebig's suggestion that gypsum should be added to manure heaps to fix the ammonia. Unfortunately there is no record of the results[101].
The most important discovery from an industrial point of view, due to his knowledge of Geology, was undoubtedly that of the phosphate nodules known in the trade as "Coprolite," at Felixstowe in 1843, when he and his family were staying there. The cliffs are formed of "London clay," topped by the "Red Crag," between which is a bed of rolled, brown pebbles, once, with the crag, forming an ancient beach. Where the white "Coralline[102]" Crag occurs, the pebble bed lies below it. This accounts for the fact that it contains remains of Miocene animals, such as teeth of the Hipparion, or ancestor of the horse.