BETWEEN GEOTROPISM AND GROWTH. 219 
zation of the root, that it is incapable of reacting to the stimulus 
of gravitation in the proper manner. Or (2) it may be supposed 
that it is not the effect of the operation per se, but the loss ofthe 
punctum vegetationis which produces the effect. These two points 
of view may be made clearer by an example: an animal may be 
prevented from performing a normal movement either by a severe 
operation which disturbs its organization, or by the loss of 
an organ which is necessary to the performance of the movement. 
A man's movements may be rendered irregular either by a general 
shock, or by blinding him. A limb may be rendered useless 
either by a violent blow or by eutting the nerve. 
We hold the second of the above theories; and believe that 
the punctum is the part of the root on which the force of gravity 
acts, and that an influence is thence transmitted to the part of 
the root which bends. According to this view, when a root is 
placed horizontally for some time before the tip is removed, the 
stimulus is transmitted to the bending-place, aud the loss of the 
tip could not therefore be expected to prevent the geotropic 
curvature taking place. 
This view has recently been criticized by Wiesner*. He believes, 
in the first place, that the hindrance to geotropism is not so well 
marked as we supposed; and, secondly, that any effect produced 
is explained by the fact that roots whose tips have been removed 
do not grow so quickly as normal roots. It is a well-known fact 
that any thing which interferes with growth interferes with 
geotropism. Sachs has shown that roots cultivated in damp 
air grow less vigorously, and are less geotropically sensitive than. 
those cultivated in damp eartht. Thus Wiesner’s view appears 
to be in accordance with known facts. But Wiesner does not 
give sufficient weight to the experiment in which the root is sub- 
jected to the stimulus of gravitation before the operation ; and 
this observation can only be brought into accordance with his 
view by supposing that the operation has different effects on the 
receptivity to stimulation and on the completion of the resulting 
curvature. It may be noted, in passing, that this would not 
agree with Wiesner’s views as to the action of other stimuli 
on plants; for he takes pains to show £ that the conditions 
* ‘ Das Bewegungsvermögen der Pflanzen,’ 1881, p. 97. U 
* ‘Arbeiten,’ i. p. 447. Sachs is careful to add that this difference in geotro- 
pism is not connected with the difference in growth. : 
+ Monograph on “ Die heliotropisehen Erscheinungen," pt. 1, p. 68. 
T 2 
