134 
Mr Darwin , Preliminary note on the function 
Preliminary note on the function of the root-tip in relation to 
geotropism. By Francis Darwin. 
[. Received 7 March 1901.] 
In my paper 1 “ On Geotropism and the Localisation of the 
Sensitive Region ” I described a new method of testing the point 
of view brought forward in the Power of Movement in Plants that 
in certain geotropic parts of plants the apex is a percipient organ 
while the more basal motor region is set in action by an influence 
transmitted from the sensitive region. 
The present paper is an attempt to apply this method to the 
case of roots 2 . 
If a seedling bean is supported by its cotyledons, the root 
projecting freely in damp air in a horizontal position, a downward 
curvature soon begins which continues until the apex points 
vertically downwards, when growth continues in the vertical line. 
According to the theory set forth in the Power of Movement in 
Plants , the tip of the horizontal root is stimulated by the force of, 
gravity and an influence is conveyed to the motor part of the root, 
in which a curvature consequently takes place : when however the 
tip is vertical it is no longer stimulated by gravitation and there- 
fore curvature ceases. If instead of fixing the cotyledons and 
leaving the tip of the root free, the arrangement is reversed, so 
that the tip is fixed in a horizontal position and the cotyledons 
are free to move, the result should according to the theory be 
quite different. For the downward curvature of the root does not 
in this case alter the position of the tip, which remains horizontal 
and should therefore continue to transmit an influence to the 
motor region. In the case of the apogeotropic hypocotyls of 
Setaria and Sorghum and of the cotyledons of Phalaris , this has 
been proved to be the case, and the remarkable coils and spirals 
so produced are figured in the paper above quoted. To apply the 
method to roots is a matter of some technical difficulty ; the tip of 
the root (e.g. in the bean) is a smooth and slimy cone and is not easily 
1 Annals of Botany, Dec. 1899. 
2 The Pfeffer-Czapek method is generally held to have proved the truth of the 
theory in question, but Wachtel’s paper shows that the technical difficulties of 
repeating Czapek’s experiment are not insignificant, so that a new method of attack 
is not superfluous. See Pfeffer, Annals of Botany, 1894: Czapek, Pringsheim’s 
Jahrb., 1895, 1900: Wachtel, Bot. Zeitung, 1899 (abstract of the original Russian 
paper). 
