Francis Darwin. 
38 
been criticised by Wachtel 1 and by Richter 2 who have failed to 
confirm them. It is undoubtedly a striking fact that the two 
observers who have seriously attempted the repetition of Czapek’s 
work have obtained other results. Nevertheless it seems to me 
impossible to give an explanation of Czapek’s experiments other 
than his own. I therefore believe that a confirmation of this 
interesting research will be forthcoming. In the case of apogeotropic 
organs not much is known about the localisation of perception. 
Czapek 3 and Rothert 4 have shown reason to suspect that the tips 
of certain grass seedlings are specialised in this way, but this 
cannot be said to be demonstrated. In 1899 s I attempted to show that 
the cotyledon is the gravi-sensitive region in Setaria and Sorghum. 
The cotyledon is pushed into a horizontal tube, so that the seedling 
is supported by the organ which is suspected of being the seat of 
perception. The hypocotyl curves upwards, but since the supposed 
sensitive region remains horizontal there is no reason why geotropism 
should cease and the hypocotyl ought therefore curl into a spiral. 
This is what actually happens, and the result still seems to me to 
be strongly in favour of the belief that the cotyledon is the seat 
of gravi-perception. 
It has been objected that this experiment does not exclude the 
existence of some gravi-perception in the hypocotyl. This is 
true, but if such sensitiveness exists it is clearly weak in comparison 
to gravi-sensitiveness of the cotyledon. 
It should be noted that there is no complete division of labour 
as regards movement, for when the cotyledon is only partly 
contained in the glass tube, the free basal portion curves markedly 
upwards. 6 The same thing occurs in a slight degree with normal 
seedlings (supported by the grain), for the first perceptible 
geotropic curve occurs in the cotyledon, which, however, afterwards 
becomes straight. 
There are several points in my experiment which require 
1 Bot. Zeitung, 1899 
2 Inaug. Diss. Freiburg i/B. (Wien, 1902). 
5 Pringsheim’s J. 1898, p. 254. 
4 Cohn’s Beitrage VII., p. 187. 
6 Annals of Botany, XIII. p. 567. 
e This may perhaps be compared to Wachtel’s observation given 
in the Bot. Zeitung, 1899, p. 231. He forced maize roots to 
grow through straight horizontal tubes open at both ends, and 
as the tip emerged it bent down when only one or two mm. 
were free. Thus curvature occurred in an abnormal part of 
the root under conditions of constraint. 
