Diatrofiism . 1 21 
through 180° without disturbing its horizontal line of growth 1 . It is 
in fact a matter of indifference to the plant whether the starch 
grains lie on AB or on CD. 
If a diageotropic organ is placed obliquely with its apex 
upwards it corrects the change by curving downwards, and in the 
same way curves upwards if its apex is made to point obliquely 
downwards. The figure gives the position of the statolith-starch' 
in the two cases. 
B represents the basal end of the cells, A the apical end. 
In i. in which the apex points upwards the starch rests on the 
basal wall B and on the basal end of the lateral wall. This 
combination of pressure may be conceived as differentiating i. from 
ii. in which pressure is on the apical end of the cell and on the apical 
part of the lateral wall. 
If we imagine the organ placed vertically with apex upwards 
there will be pressure on the basal wall, that is there will be a state 
of things somewhat resembling position i. of the figure, but there 
will be nothing to suggest a curvature in one direction more than 
another since the pressure is identical in the two lateral walls. The 
same thing must be true if the apex points vertically downwards so 
that for diageotropism there should be two positions of labile 
equilibrium as Noll long ago pointed out. 2 This turns out to be 
true : if a diageotropic organ is kept vertical (by being placed in a 
glass tube) it is not stimulated, and shows no after effect 
when released from the tube. 3 The lability of the equilibrium is 
shown by the fact that a diageotropic organ placed vertically 
without being controlled by means of a tube does not remain vertical. 
This is because a free organ can by circumnutation escape from 
strict verticality and as soon as this happens a stimulus arises 
tending to increase the angular divergence from the vertical. 
Diageotropism has been largely studied in lateral roots ; these 
1 Elfving, Sachs’ Arbeiten ii. 
2 Heterogene Induction, p. 37. 
3 Czapek, Pringsheim’s Jahrb. 32, p. 243. Noll, ibid 34, p. 473. 
