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 Poiver 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) . 



