INFLUENCE OF EXTERNAL CONDITIONS ON GROWTH 293 



grow upward and that roots grow downward are such obvious facts that they 

 remained uninvestigated for a long time. The first author to give this difference 

 serious attention was Dodart, 1 and much work has been published in this con- 

 nection since his time, but no real insight into these phenomena has even yet 

 been obtained. 



If a growing plant is changed from the vertical to the horizontal position, 

 the root-tip soon bends downward and the tip of the stem upward. Knight 2 

 showed that this bending of growing plant organs is due to the influence of the 

 force of gravitation. Seeds were allowed to germinate while attached to a rapidly 

 rotating wheel. The axes of the seedlings assumed positions in the radii of 

 the rotating disk, all of the main roots directing their tips outward while the 

 tips of the main stems were directed inward. Here the force of gravitation was 

 not allowed to act continuously upon the seedlings in any particular direction 

 (since the axis of the wheel was horizontal), and in place of this force as it 

 usually acts on plants was substituted the centrifugal force generated by rota- 

 tion. The primary roots, which usually elongate in the direction of the pull of 

 gravitation, now grew in the direction of the centrifugal pull; that is, toward the 

 circumference of rotation. The primary stems, which usually direct their tips 

 away from the center of the earth, grew in the direction opposite to that of the 

 centrifugal pull; that is, toward the center of rotation. 



The phenomenon of bending in response to the force of gravitation is termed 

 geotropism. When the organ bends so as to direct its tip toward the center of 

 the earth its geotropism is said to be positive, and when the bending occurs in 

 the opposite direction it is said to be negative. Primary stems are generally 

 negatively geotropic and primary roots are generally positively so. 



The geotropism of lateral branches of both shoots and roots is less pro- 

 nounced; these organs generally do not assume the vertical position, but take 

 an oblique direction, more or less nearly approaching the horizontal. [They 

 are said to be apo geotropic or plagiotropic] 



For the removal of the one-sided geotropic stimulus in experiments, various 

 forms of clinostat are used, as well as the centrifuge already mentioned. The 

 Pfeffer clinostat (Fig. 139) consists essentially of a metal axis (c) rotated by a 

 clock-movement (a) and bearing at its free end the objects of the experiment. 

 The axis may be arranged so as to have any desired position, horizontal, ver- 

 tical, etc., the clock being correspondingly tilted and fastened by the screw n. 



If a cork disk (/, Fig. 139, B) bearing germinating seeds is attached to the 

 horizontal axis of a clinostat, with its plane surfaces perpendicular to the axis, 

 and slowly rotated, the seedlings do not bend, but continue to grow in whatever 

 direction they may have had when attached. The force of gravitation is of 

 course not prevented from acting upon the plants in such a case, but the direc- 

 tion of this force is continually varied, so that during each revolution the gravity 



1 [Dodart, [ .], Sur l'affectation de la perpendiculaire remarquable dans toutes les tiges, dans plusieurs 

 racines, et autant qu'il est possible dans toutes les branches des plantes. Hist. Acad. Roy. Sci. 1700 

 (2nd ed.) : 47-63- Paris, 1741-] 



2 [Knight, Thomas Andrew, On the direction of the radical and germen during the vegetation of seeds. 

 Phil, trans. Roy. Soc. London 1805 tPart I) [96]: 99-108. 1806.] 



