Knight, Thomas Andrew

Thomas Andrew Knight, (born Aug. 12, 1759, Ludlow, Herefordshire, Eng.—died May 11, 1838, London), British horticulturalist and botanist whose experiments on the adaptive responses of plants and the changes in direction of stem and root growth were the basis of later work on geotropisms.

After graduating from the University of Oxford, Knight applied scientific principles and techniques to practical horticultural problems. He became a correspondent with Sir Joseph Banks of the Royal Society. Knight’s scientific work was originally published in the Royal Society’s Philosophical Transactions (1795–1814); a volume of collected papers appeared in 1841.

Knight performed experiments on the ascent and descent of sap in plants. His experiments with ringing (girdling) disproved Stephen Hales’s theory that bark is formed from alburnum (xylem). He also showed that sap in the outer bark differs from the nutrient-rich sap of the alburnum. In 1806 Knight’s famous letter to Banks described his experiments with germinating seeds, which used centrifugal force to show the effects of gravity on the growth of plants. Knight unsuccessfully used mechanical physical properties to explain the seeds’ adaptive responses.