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? 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 
(Setaria, 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, eg. splitting the root longitudinally’, 
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? 
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, vu. 1894. 
2 See F. Darwin, Linnean Soc. Journal (Bot.) x1x. 1882, p. 218. 
3 See his excellent summary of the subject in Flora, 1894 (Ergaénzungsband), 
p. 199. 
