573 
Localization of the Sensitive Region. 
needs further experimentation. Another point is worth 
mention, though here again more work is wanted. If a 
seedling is fixed by the tip in a vertical glass tube, the apex 
of the cotyledon being upwards, there seems no reason why 
curvature should occur, since the sensitive part, the cotyledon, 
is in the position of non-stimulation. But curvature does 
occur in this situation, and this may be due to two causes : 
(i) the tube can only be approximately vertical, and it is 
possible that the slight stimulus (which analogy suggests 
must accompany slight obliquity) being continuous may be 
enough to make the hypocotyl bend ; (2) it is possible that 
a certain amount of geotropic sensibility may exist in the 
hypocotyl ; if this were so, it would behave like an ordinary 
apogeotropic organ, and these are well known to curve up 
when fixed with all available accuracy, with the free end 
down b But this would only account for a movement 
through 180 0 , which would bring the free end vertically 
upwards. In many of my preliminary experiments the tubes 
were certainly not strictly vertical ; and when especial care 
was directed to this point, the effect was small, as the following 
observations show. Seven Sorghum seedlings were fixed by 
their upwardly directed cotyledons, and after two days only 
gave an average deviation of about 30° from the vertical. 
The tubes holding the cotyledons were then fixed horizontally, 
and by the next day six out of the seven had curved through 
180 0 more or less. 
Theoretically the method here described is an ideal one for 
discovering in what position a geotropic organ is most strongly 
stimulated by gravitation. Sachs 2 , as is well known, supposed 
that the horizontal is the position of maximum stimulation, a 
conclusion which has been controverted by Elfving 3 and 
Czapek 4 . In 1888 Miss Bateson and 1 5 pointed out that as 
1 Frank, loc. cit., p. 80. Czapek, however, shows that if a geotropic organ 
is placed in gypsum, so that it cannot circumnutate, it is not geotropically 
stimulated when the apex is vertical. 
2 Arbeiten, i, p. 454 ; ii, p. 240. 3 Acta Soc. Sc. Fenn., 1880. 
4 Czapek, Prings. Jahrb. xxvii, 1895; xxxii, 1898. 
5 Annals of Botany, 1888, p. 65. 
