On the Direction of the Radicle and Germen during the Vegetation of Seeds. In a Letter to... Joseph Banks. Read January 9, 1806.

(London, W. Bulmer and Co., 1806). 4to. No wrappers as extracted from "Philosophical Transactions" 1806. With titlepage to 1806, Part I. Pp. 99-108. With a stamp on verso of titlepage.

First apperance of a milestone paper in plant physiology as the concept of tropistic behaviour arose from the experiments described in this paper. Knight showed that forces, generated by means of a water-wheel, made roots and shoots of seedlings orient themselves to centrifugal forces, just as they do to gravity.

Knight's investigations on geotropism "enabled Knight to distinguish between geotropic and hydrotropic reactions of the root, and opened the whole question of tropic responses, of their physical basis and adaptive significance in the lfe of plants."(Morton, "History of Botanical Science", p. 390.).

"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."(DSB).

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