Francis Darwin. 
126 
If a plant is placed upright in the dark until the leaves 
point vertically upwards and is then placed on the klinostat in the 
position shewn in the figure (A) the leaves bend backwards until 
they reach position B. The beginning of this curvature looks like 
apheliotropism, but if it were so the leaves would continue to bend 
until they reached position C. In the same way if the experiment 
begins with position C the leaves curve toward the light, they do 
not however continue to do so until position A is reached, but as in 
the first experiment they stop at position B. Since these leaves 
reach the diageotropic position by either bending to or from the 
light they cannot be called positively or negatively heliotropic. 
The striking feature about them is that the curvatures cease when 
the leaf reaches the light.position, and it is this fact which gives 
us the right to call the movement diageotropic. Some plants, for 
instance seedling cherries, are unable to assume the light-position 
on the klinostat 1 : in my experiments they took up a position 
resembling C in the figure. This fact may be fairly quoted in favour 
of De Vries’ theory. It is however possible to look on the loss of 
gravitation not as the loss of one of the elements which strike a 
balance, but rather as the absence of a condition to which 
the plant has become habituated. Be this as it may there can be 
no doubt, in the light of researches more modern than my own, 
that Frank’s view of diageotropism is broadly speaking the correct 
one. I refer especially to Vochting’s important paper 2 where he 
showed by means of the klinostat and by the simpler method of 
inverting his plants and illuminating them from below, that light 
may be the determining factor quite independently of gravitation. 
1 F. Darwin loc. cit. In the same way Fischer (Bot. Zeitung, 
1890) showed that some plants can execute sleep-movements 
on the klinostat whilst others cannot. 
* Bot. Zeitung, 1S88. 
( Conclusion ). 
