DOWNWARD CURVATURE DUE TO GEOTROPISM. 6gi 



curvature is most pronounced in the region i, 2, 3, where also most elongation 

 has taken place. By means of the curvature, again, the piece 1-2, and still more 

 the piece o-i, has come into a position where it is directed nearly vertically down- 

 wards, and where the active component of the force of gravity can exert but a very 

 feeble geotropic influence; the very feeble curvature obtained at the beginning 

 6f the part 1-2 is thus no longer appreciably increased, whereas the part 2-3 iri 

 consequence of its more favourable position, in which it is still cut by the direction 

 of gravity at a tolerably large angle, becomes still more curved. From all that 

 has been said, it will be noted that the youngest part o-i becomes directed down- 

 ward chiefly passively, by means of the curvature of the parts lying behind it, 

 and when it has attained this vertical direction it simply goes on growing in it. 



The careful observation of numerous roots by this and other methods leaves 

 no doubt that the geotropic processes are here in all essential points the same 

 as in the up-curving of shoot-axes, only they take place in exactly the opposite 

 direction, and on this account it is quite correct to distinguish the two phenomena 

 by means of the terms positive and negative. Moreover, as I have demonstrated, 

 the down-curving of roots agrees with the up-curving of shoot-axes in" that a re- 

 tardation of the elongation of the axis of growth occurs during the curvature, while 

 the side which will be convex grows more strongly and that which will be concave 

 more feebly than would be the case in the vertical direction. 



The proof of the identity of the processes resulting in positive and negative 

 curvature which I brought forward, was important because not only Knight but 

 Hofmeister also had referred the down-curving of roots to essentially other causes 

 than those to which the up-curving of shoot-axes is due. In particular, it was 

 believed that the down-curving of the root must be regarded as the mere sinking 

 of a viscous pasty mass. This view was opposed by Frank, among others ; and it 

 is, in fact, utterly erroneous, as can be demonstrated at once by compelling the 

 down-curving root to set in motion a weight much greater than its own. For 

 this purpose I have placed vigorous roots horizontally and with their apices sub- 

 merged in a small shallow vessel full of water, from which a thread, suspending a 

 weight of i-i"5 gr., was carried over a pulley. Should the root-apex curve down- 

 wards it must set the weight in motion ; and this actually occurred. The experiment 

 illustrated in Fig. 390 is simpler and more elegant,, but is of course less obvious : the 

 result, however, is the same. A seedling of the Broad Bean ( Vi'a'a Fahct), the root and 

 plumule of which were perfectly straight, was fixed into the piece of cork k by means of 

 a needle, so that the root-apex lay horizontally on the surface of the mercury, figured 

 black: nn denotes a thin layer of water on the mercury. After about 24 hours the 

 seedling had taken the form represented in the figure. The plumule with its sharply 

 curved bud had become directed vertically upwards; but the tip of the root had 

 become directed vertically downwards in the usual way, having meanwhile 

 penetrated the mercury (which is about 13-6 times as heavy as the watery substance 

 of the root) to the depth of about r cm. Thus the part which had penetrated the 

 mercury had displaced a weight 13-6 times as great as its own. It is obvious that 

 the weight of the mercury presents an equivalent resistance to the entrance of the 

 root-apex, and this makes itself evident in the figure by a curvature resembling 

 a drawn out .S" being produced behind the curve due to geotropism. More- 



Y y 2 



