THE MOVEMENTS OF PLANTS. 
195 
a permanent obstruction to a radicle, causes the latter to 
deviate from its path till it finds no great resistance ; and 
Mr. Darwin observes that this is the only-known instance of 
an organ turning away from an irritant. By fixing cards to 
one side of the tips of vertically growing radicles, they were 
caused to tend upwards, as if trying to escape from the cards, 
sometimes even forming complete circles, and in one case a radicle 
actually tied itself into a knot (Pl. Y. fig. 3*). The tip in 
the act of forming a loop, generally rubs against the upper 
part of the radicle and pushes off the attached card ; the loop 
then contracts or closes, but never disappears. The apex after- 
wards grows vertically downwards (fig. 2). This sensitiveness 
is confined to the tip for a length of from 1 mm. to 15 mm., 
while the upper adjoining part of the radicle, for a length of 
from 6 or 7 to even 12 mm., is excited to bend away from the 
side which has been irritated. After a time the radicle 
apparently becomes accustomed to the irritation, as occurs in 
the case of tendrils, and the irritant no longer affects its 
downward growth, which is resumed. 
After a radicle, which has been deflected by some stone or 
root from its natural downward course, reaches the edge of the 
obstacle, geotropism will direct it to grow again straight down- 
wards ; but geotropism is a weak force, and the radicle assists 
it by having its upper part, a little above the apex, sensitive to 
contact, but acting in a directly opposite manner to that of the 
tip, for it causes the radicle to hend like a tendril towards the 
touching object, so that as it rubs over the edge of an obstacle 
it will bend downwards. This downward bending coincides 
with that due to geotropism, and both will cause the root to 
resume its original course. 
Finally, Mr. Darwin observes,! the several co-ordinated 
movements by which radicles are enabled to perform their 
proper functions are admirably perfect. In whatever direction 
the primary radicle first protrudes from the seed, geotropism 
guides it perpendicularly downwards ; the radicles emitted 
from the primary are, however, only acted upon by geotropism 
in such a manner as to cause them to bend obliquely down- 
wards, unless the end of the primary radicle be cut away, then 
the secondary ones grow vertically downwards. The tertiary 
radicles are not influenced by geotropism ; hence all the rootlets 
grow in the most advantageous manner, and the whole soil is 
thus closely searched. 
Hypocotyls and Epicotyls. — The radicle having penetrated 
* The figure of the radish is not from Mr. Darwin’s work, but from a 
drawing made many years ago, and which would seem now to have received 
its explanation. 
t P. 196. 
