468 LECTURE XVIII. 



to the growing zones behind it in which the curvature takes 

 place. 



The assertion by Darwin of the localisation of geotropic 

 irritability in the tips of roots has given rise to a number of 

 researches on the subject the results of which are conflicting. 

 The cause of this conflict is the difficulty of ensuring the 

 normal growth of roots under conditions favourable for ob- 

 servation, and the fact, to which Sachs first drew attention, 

 that the removal of the tip causes roots to undergo the most 

 various curvatures. This irregularity is due, as Darwin and 

 Kirchner state, to an oblique, that is not exactly transverse, 

 "decapitation", a curvature taking place towards the longer 

 side of the injured root, but Detlefsen replies that when roots 

 are vigorous, irregular nutations follow decapitation even when 

 the section is as nearly as possible transverse. Some observers, 

 such as Sachs and Detlefsen, deny altogether that decapitated 

 roots are incapable of geotropic curvature. Wiesner admits 

 that the capability of performing geotropic curvature is 

 diminished by decapitation, and ascribes the diminution to a 

 diminished turgidity of the growing cells. He finds also that 

 a decapitated root grows in length less rapidly, when in moist 

 air, than an uninjured root an observation which has been 

 confirmed by Molisch though it grows more rapidly when in 

 water. On the other hand Darwin's view is supported by the 

 observations of F. Darwin, Kirchner, Krabbe, and Brunchorst. 

 F. Darwin finds, in repeated experiments, that decapitated 

 roots do not curve geotropically, and that decapitation does 

 not so diminish the activity of growth in length of a root 

 as to account for its loss of geotropic irritability. Kirchner 

 also denies that there is any such difference in the rate of 

 growth in length of normal and decapitated roots as Wiesner 

 and Molisch assert, and points out that, even admitting this 

 to be the case, geotropic curvature is not proportional to 

 the rate of growth in length. From Krabbe's observations 

 it would appear that decapitated roots grow in moist air 

 rather more rapidly than normal roots. Brunchorst has 

 made the very remarkable observation that if the com- 

 munication between the region of curvature and the tip of 



