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 (Rothert, Cohn's "Beitrage",
VII. 1894.) 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 gravitational
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 (See F. Darwin, "Linnean
Soc. Journal (Bot)." XIX. 1882, page 218.), 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 (See his excellent
summary of the subject in "Flora" 1894 (Erganzungsband), page 199.) 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 par