122 
Francis Darwin. 
grow at various angles with the vertical, so that they may be either 
horizontal or may point either obliquely upwards or downwards. 
In spite of this fact they may conveniently be termed diageotropic 
since they do not grow vertically, and whatever their angular 
direction may be they have the capacity of returning to it if displaced. 
This may be briefly expressed by saying that in the case of lateral 
roots the position of stable equilibrium varies. 
Sachs’ views as to diageotropism differed from those here given. 
He held that lateral roots are pros-geotropic but not sufficiently so 
to produce vertical growth, their sensitiveness being so feeble that 
they cease to react to gravity at a slight degree of obliquity below 
the horizontal. This hypothesis need not concern us as it is in¬ 
consistent with the fact that roots curve upwards when displaced 
below their normal line of growth. 1 should not have mentioned 
this difficulty were it not for the existence of certain reactions 
which suggest the co-existence of pros- and dia-geotropism in lateral 
roots. Some very familiar facts have been held (incorrectly as it 
seems to me) to be evidence for this point of view. When the main 
root of a bean is removed one of secondaries takes its place and 
grows vertically downwards. Since this result fails when the plant 
is grown on the klinostat, it is certain the downward curvature of 
the secondary root is a prosgeotropic not an autogenic curvature. 
The experiment merely shows that under new conditions ( i.e ., the 
amputation of the primary root) the secondaries react in a new way 
to gravitation, I can see no reason why it should be held to prove 
that before the operation pros-geotropism was in any way concerned 
in the reaction of the normal secondaries. The same may be said 
of the fact that illumination alters the angle at which secondary 
roots grow. 
Czapeld has shown that lateral roots exposed to strong centri¬ 
fugal force curve away from the centre of rotation ; they do not 
however, reach the line of the radius, but come to rest some 30" 
from it. This certainly looks like pros-geotropism : but if we 
compare the results with the facts of heliotropism our conclusion 
will be shaken. It is well known that Phyconiyces is heliotropic 
under moderate illumination, but becomes apheliotropic with a 
strong light. But this has never been held to prove that Phyconiyces 
is at the same time positively and negatively heliotropic. Another 
property of lateral roots is that if they are placed obliquely above 
1 On the general question of the pros-geotropism of lateral roots 
see Czapek’s interesting paper in Sitzb. k. Akad. Wien, 
Band 104, p. 1197. 
