THE SEEDLING AND YOUNG PLANT 35 
pressed that they form 
mere flattened 
many times broader than 
they are thick (fig. 8). 
It has already been 
mentioned that the tip of 
the young primary root 
circumnutates, and Dar- 
win also found that the 
tip of the radicle is ex- 
tremely sensitive to the 
sheets 
irritation of small bodies 
in contact with it. It is 
also positively geotropic, 
directing itself vertically 
downwards if the partially 
grown radicle is laid hori- 
zoutally ; and it may be 
assumed from the _ be- 
haviour of other plants of 
the same kind that the 
tip of the radicle is nega- 
tively heliotropic—i.e. it 
turns away from the 
source of light. Whether 
it is also sensitive to dif- 
ferences in the degree of 
moisture on different sides 
(hydrotropic), or to differ- 
Fia. 8.—Portion of an older root 
of an oak, which had pene- 
trated while young between 
two pieces of hard rock, and 
had to adapt its form accord- 
ingly as it thickened. (After 
Dobner.) 
D2 
