Growth Movements of the Radicle of Striga

Abstract

THE directional growth of the radical (haustorium) of the root parasite Striga asiatica (= S. lutea) is normally controlled by a chemotropic factor diffusing from the host plant root¹. The parasite seeds may be germinated by treatment with a solution in which the host roots (Sorghum vulgare, etc.) have been growing. In this case the radicle undergoes a fixed growth movement which is roughly spiral. A basically spiral movement is seen in about 40 per cent of seedlings. More usually the movement is reduced to a strong curvature in one plane followed by a period of more or less straight growth, or by apparently anomalous curvatures in any direction. By growing the seedlings in agar cultures, it was shown that the movements were not influenced by the orientation of the seedlings to gravity or by rotation on a klinostat, at least during the initial curvature, although with forces greater than gravity (up to 10g) a positive movement was superimposed on the fixed growth movement.