Root-tip 393 



opaque caps on the tips of seedling grasses (e.g. oat and canary- 

 grass) and then exposing them to light from one side. The difference, 

 in the amount of curvature towards the light, between the blinded 

 and unblinded specimens, was so great that it was concluded that 

 the light-sensitiveness resided exclusively in the tip. The experiment 

 undoubtedly proves that the sensitiveness is much greater in the tip 

 than elsewhere, and that there is a transmission of stimulus from the 

 tip to the region of curvature. But Rothert x has conclusively proved 

 that the basal part where the curvature occurs is also directly 

 sensitive to light. He has shown, however, that in other grasses 

 (Setarla, Panicum) the cotyledon is the only part which is sensitive, 

 while the hypocotyl, where the movement occurs, is not directly 

 sensitive. 



It was however the question of the localisation of the gravita- 

 tional sense in the tip of the seedling root or radicle that aroused 

 most attention, and it was on this question that a controversy arose 

 which has continued to the present day. 



The experiment on which Darwin's conclusion was based consisted 

 simply in cutting off the tip, and then comparing the behaviour 

 of roots so treated with that of normal specimens. An uninjured 

 root when placed horizontally regains the vertical by means of a 

 sharp downward curve ; not so a decapitated root which continues 

 to grow more or less horizontally. It was argued that this depends 

 on the loss of an organ specialised for the perception of gravity, and 

 residing in the tip of the root ; and the experiment (together with 

 certain important variants) was claimed as evidence of the existence 

 of such an organ. 



It was at once objected that the amputation of the tip might 

 check curvature by interfering with longitudinal growth, on the 

 distribution of which curvature depends. This objection was met 

 by showing that an injury, e.g. splitting the root longitudinally 2 , 

 which does not remove the tip, but seriously checks growth, does 

 not prevent geotropism. This was of some interest in another and 

 more general way, in showing that curvature and longitudinal growth 

 must be placed in different categories as regards the conditions on 

 which they depend. 



Another objection of a much more serious kind was that the 

 amputation of the tip acts as a shock. It was shown by Rothert 3 

 that the removal of a small part of the cotyledon of Setaria 

 prevents the plant curving towards the light, and here there is no 

 question of removing the sense-organ since the greater part of the 



1 Rothert, Cohn's Beitrage, vn. 1894. 



2 See F. Darwin, Linnean Soc. Journal (Bot.) xix. 1882, p. 218. 



3 See his excellent summary of the subject in Flora, 1894 (Erganzungsband), 

 p. 199. 



