396 EFFECT OF GRAVITY UPON GROWTH [Ch. XV 



ing at the blind end. Under these conditions no geotropic cur- 

 vature occurred ; but such curvature always took place when 

 the root tip was placed horizontally or at any acute angle with 

 the horizon. This experiment seems, then, better fitted than 

 any previous one to prove that the geotropic sensitiveness of 

 the root resides in the apex.* Consequently, since, as an in- 

 spection of Fig. 106 will show, the curving part of the root can 

 contain none of the originally irritated cells, there must be a 

 transmission of stimuli from the root tip to the curving region. 



Two associated phenomena remain to be considered. First, 

 geotropic response is more powerful, the bending becomes 

 stronger, as the angle made by the root with the vertical in- 

 creases, reaching a maximum when the root is horizontal 

 (Sachs, '73, p. 454). Secondly, as already indicated, response 

 does not take place immediately after the root is placed hori- 

 zontally. In one experiment of Sachs' ('73, p. 440) a bean 

 radicle placed horizontally and growing in loose earth at 20° C. 

 began to turn downwards in the second hour. There is thus a 

 considerable latent period. 



The cause of geotropism in roots is indicated by the con- 

 ditions of its occurrence already mentioned. It is intimately 

 associated with growth, yet, as Kirchnee ('82) has shown, it 

 may occur at a temperature (2° to 3.5° C.) at which growth is 

 extremely slow. The turning is clearly due to unequal growth 

 upon the convex and concave sides of the root. But is it due 

 to acceleration on the convex side over the normal, to retarda- 

 tion on the concave side, or to both? Measurements have been 

 made upon plants to decide this question. In one set of such 

 measurements made upon the bean radicle by Sachs ('73, 

 p. 463) the growth was hastened about 3% on the convex side 

 and retarded 42% on the concave side, so that both accelera- 

 tion and retardation occur. 



Finally, lateral roots which run more or less obliquely down- 

 wards have been shown by Sachs ('74) and others to be 

 influenced by gravity; for, if the plant be inverted, the roots 

 will turn until they assume their normal inclination to the 

 horizontal plane. 



* But species may differ in this respect as in phototropism (see page 441). 



