Knight, Thomas Andrew
Knight, Thomas Andrew
(b. near Ludlow, Herefordshire, England, 12 August 1759; d. London, England, 11 May 1838)
T. A. Knight was born into an old Shropshire family with independent means and a substantial estate. His father, Thomas Knight, died when he was young; his elder brother, Richard Payne Knight, the numismatist, helped him by establishing him at Elton with a farm and hothouses, and later by handing over to him the management of 10,000 acres at Downtown Castle, his home in Herefordshire.
As a young man, Knight had some education at Ludlow grammar school, but he learned more by observing and asking questions in the gardens at his home. He graduated at Balliol College, Oxford, and in 1791 married Frances Felton. They had a son who died in youth and three daughters. As a result of his early experiments on breeding fruit, vegetables, and cattle, he was recommended by his brother to Sir Joseph Banks as a correspondent to the Board of Agriculture. He met Banks in London in 1795 and began a correspondence in which he recorded his most important scientific work, read by Banks to the Royal Society. He was elected to the Royal Society in 1805. When the Horticultural Society was founded in 1804 Knight was a member, and from 1811 he was its president.
The main impetus to Knight’s research was practical: He wanted to improve the culture and yield of produce from farm and garden. His most important contribution to horticulture and agriculture was in the application of scientific principles and techniques to practical situations of the grower or breeder. He worked on the design of hothouses, control of pests, and cider making as carefully as he did on theoretical research. His first published letter to Banks (1795) was on the gradual decline of stock propagated by grafting and the need to develop new varieties from seed, particularly by cross fertilization. The latter would produce both greater vigor and a wider range of offspring from which the most useful could be chosen. He wrote in 1799; “In promoting this sexual intercourse between neighbouring plants of the same species, nature appears to me to have an important purpose in view.”
For practical purposes he bred apples, pears, and Herefordshire cattle, but he also worked from 1787 on peas selected as suitable because they were annuals with clearly differentiated paired characteristics. In a paper of 1799 he described how in the crossing of a gray pea with a white pea all the first generation are gray, but the white reappear in the second generation. He had observed dominance, but had done none of the careful statistical work that made Mendel’s experiments of half a century later so significant.
Study of the developing fruit led him to design careful experiments on the translocation of sap. He used colored infusions to trace the ascent of sap through the outer layers of laburnum (xylem), branching to petioles and passing to the developing fruit; he also used ringing to trace the descent of sap, disproving Hales’s theory that bark was formed from the alburnum. He believed that the sap circulated, but showed that sap in the bark differs from aqueous sap in the alburnum, as a result of nutrients received from the leaves. Sap which is carried to leaves and air “seems to acquire (by what means I shall not attempt to decide) the power to generate the various inflammable substances that are found in the plant. It appears to be brought back again … to add new matter.” If leaves were shaded the quantity of alburnum deposited was small. He discussed at length the forces causing this circulation, certain only that the explanation must be mechanical. He knew of capillarity and demonstrated transpiration and observed the spiral vessels, but he took no account of root pressure or cohesion of liquid columns, so he suggested a process involving the contraction and expansion of “silver grains.” Sap descended, he thought, by gravitation.
His most famous work was on what are now called geotropisms. In a letter read by Banks to the Royal Society in 1806, Knight described how he eliminated the influence of gravitation on germinating seeds: He attached them at various angles to the rim of a vertical wheel which was driven by a stream in his garden to revolve continuously at a rate of 150 r.p.m. As the germinating plants grew, each shoot was directed to the center of the wheel; when a shoot passed the center of the wheel its tip turned back so that growth was still centripetal; the roots grew away from the center. Next he set up a similar structure with the wheel horizontal and rotating at 250 r.p.m. so that the seedlings were influenced by both gravitation of the earth an the centrifugal force. In this case, growth was at an angle of 80° to the vertical, the shoot upward and inward, and the root downward and out. Reducing the rotation to 80 r.p.m. decreased the centrifugal force to such an extent that the plants grew at an angle of 45° to the vertical.
Here he ran into the philosophical problem of how the plants “perceived” the force acting on them, as he was himself “whooly unable to trace the existence of anything like sensation or intellect in the plant.” Yet these seedlings clearly reacted to gravitational force, for he had shown in 1801 that vine leaves always move so as to present the upper surface to the sun; and in 1811 he was to demonstrate that in certain conditions roots may be deflected from the vertical by moisture. The plants were making adaptive responses, but his explanation was typically mechanical: the roots bend down by their own weight, while in the shoot, nutrient sap moves to the lower side and there stimulates differential growth and curvature to the vertical. He was aware that this was not entirely satisfactory as he had not explained the “weeping” tree.
Adaptive response to the physical environment by differential growth was his only major discovery, although he came surprisingly near several others. His papers, readily accessible in Philosophical Transactions, were known to Darwin, who worked on light sensitivity in climbers as well as on variation and selection. Knight was friendly with many scientists of his day apart from Banks. Dutrochet visited him in 1827 and repeated some of his experiments. Sire Humphrey Davy quoted his experiments with wheels in his lectures on agricultural chemistry of 1802 to 1812, and included on elegant plate of the apparatus in the published version. Knight added some notes to the third edition, and in the fourth edition Davy altered the dedication from the Board of Agriculture to Knight.
I. Original Works. Knight’s scientific work was published in Philosophical Transactions of the Royal Society of London, and is listed in the Society’s catalogue of Scientific Papers. The most significant of the papers referred to in the text are “Observations of the Grafting of Fruit Trees”, 85 (1795), 290-295; “An Account of Some Experiments on the Fecundation of Vegetables”, 89 (1799), 195-204; a series of papers on sap, 91 (1801), 333-353; 93 (1803), 277-289; 94 (1804), 183-190; and 95 (1805), 88-103; and “On the Direction of the Radicle and Germen During the vegetation of Seeds”, 96 (1806), 99-108.
The above and some eighty other papers were reprinted in A Selection From the Physiological and Horticultural Papers, Published in the Transactions of the Royal and Horticultural Societies by the Late Thomas Andrew Knight, to Which is Prefixed a Sketch of His Life (London, 1841).
His two most important books are Pomona Herefordiensis (London, 1811) and A Treatise on the Culture of the Apple and Pear, and on the Manufacture of Cider and Perry (Ludlow, 1797; 2nd ed., 1801;3rd ed., 1808).
II. Secondary Sources. The most useful personal source is the Sketch of His Life, written by his daughter Mrs. Frances Acton and published in the Selection already cited. The best scientific evaluation is C. A. Shull and J. F. Stanfield, “Thomas Andrew Knight: in Memoriam”, in Plant physiology,14 (1939), 1-8. A brief obituary notice is in Abstracts of the Papers Printed in the philosophical Transactions of the Royal Society of London From 1837 to 1843 (later the Proceedings), 4 (1843), 92-93.
There is an account of his practical work in “Thomas Andrew Knight and His Work in the Orchard”, in Wool-hope Naturalists’ Field Club, Herefordshire Pomana,1 (1876), 29-46.
Darwin does not appear to have referred to Knight’s work on geotropisms, but he mentioned his work on hybrid vigor several times in The Effects of Cross and Self Fertilisation in the Vegetable Kingdom (London, 1876).He also quoted Knight’s definition of domestication in his essay first exploring the ideas of the Origin in 1844.
See also Sir Humphry Davy, Elements of Agricultural Chemistry (London, 1813, 3rd en., 1821).
There are numerous references to Knight’s place in the history of scientific thought in the standard textbooks and in biographies of his many friends. Many letters remain from his voluminous correspondence, the largest collection being in the British Museum (Natural History). Those written to Banks may be traced through W. R. Dawson, ed., The Banks Letters, a Calendar of the Manuscript Correspondence of Sir Joseph Banks (London, 1958).
A good bibliography of source material is included in the entry for Knight in A Biographical Index of Deceased British and Irish Botanists, compiled by J. Britten and G. S. Boulger, 2nd ed. (London, 1931), pp. 176-177.
Diana M. Simpkins