The Physiology of Flowering Plants - KHAM PHA MOI

332 GROWTH MOVEMENTSFig: 12:9 Centrifugal stimulationof the root tip and base in oppositedirections.The arrows a to c showthe direction of the forces acting onthe tip which projects beyond thecentrifuge axis; the arrows 1 to 5represent forces acting on the basalpart; the lengths of the arrows areproportional to the force. If 1.5–2mm of the tip is allowed to projectbeyond the axis, the root behaves asif the tip alone were stimulated,even though the forces acting on thebase in the opposite direction aremuch stronger. From Larsen(1962).Growth54321Axis of centrifugeabcgravity and is the angle relative to the vertical (Fig. 12.9). Anothermeans of controlling the gravity perceived by the plant is to rotate it.As described above, slow rotation on a klinostat has the effect ofcancelling out the directional component of the gravitational force,providing that the rate of rotation is sufficiently slow to avoid centrifugaleffects. Rotation at higher speed, in a centrifuge, allows thegravitational force experienced by the plant to be readily increased.A true reduction in the force of gravity is somewhat more challengingand requires flights on rockets on parabolic trajectories or into space.Such studies have shown that gravitropism exhibits reciprocity, i.e.the response observed is the product of the time of stimulation theintensity of stimulation: a long period of weak stimulation causes thesame response as a shorter period of more intense stimulation. Forthe oat coleoptile this relationship holds true for forces between 58.4and 0.0014 g, requiring exposure of 5 s or 68 h respectively. Thethreshold for the oat coleoptile is therefore approximately 300 g sat room temperature. Once this threshold has been exceeded themagnitude of the response generally increases with the intensity ofthe stimulus until a maximum is reached.