569 
Localization of the Sensitive Region. 
Rothert’s argument is as follows. When a heliotropically 
curving Phalaris is carefully watched, a certain relation 
between the form of the curvature and the distribution of 
growth-rate is found to exist. And since the same relation 
is found in a geotropically curving Phalaris , Rothert argues 
that this relation, being correlated with a certain localization 
of heliotropic irritability in one case, is probably correlated 
with similar localization of geotropic irritability in the other 
case. Whether or no the arguments are sound, the fact is as 
Rothert and Czapek suppose. 
Let us first take the case of an apogeotropic organ in 
which the power of perceiving the gravitation- stimulus is not 
confined to one part of the organ, but distributed through all 
the growing parts, such an organ, in fact, as Sachs describes 
as typical. Imagine two such organs placed horizontally, one 
end being supported and the other projecting freely into the 
air. Let us further imagine that one specimen (normal) is 
fixed by its basal end, while the other (inverted) is fixed by 
its apical end. What will happen ? The behaviour of both 
specimens will be the same, i. e. the free ends of both will 
bend up and will ultimately point vertically upwards. 
An experiment of this sort was made with young dandelion 
stalks by Frank 1 in 1868, from a different point of view, 
namely, to show that the geotropic curvatures are not affected 
by the morphological position of the organ, that is, by whether 
the free end is morphologically the base or the apex. Noll 2 
has made similar experiments with the flower stalks of 
Aconitum in his researches on the relation between epinasty 
and geotropism. 
All this seems to have little bearing on the question before 
us, but its connexion is real enough. Imagine that precisely 
a similar experiment is made with organs in which only the 
apex is sensitive to gravity, while the basal part is not directly 
sensitive, but merely bends upwards when it receives a trans- 
1 Beitrage zur Pflanzenphysiologie : I. Ueber die durch die Schwerkraft verur- 
sachten Bewegungen von Pflanzentheilen, p. 80, 1868. 
2 Flora, 1893, p. 360. 
