860 MOVEMENTS OF PLANTS. 
action Sachs has applied the term of revolving nutation, which 
Darwin has simplified into that of circwmnutation. So soon as 
the organ meets with a support its motion is arrested, and it 
becomes spirally twined round by the arrest of the movement of 
successive portions. Tendrils contract spirally soon after they 
have laid hold of a support, and so draw up the stem to which 
they are attached. The remaining movements belonging to this 
class have been divided by Schleiden in the following manner :— 
1. Movements which evidently depend on external influences. 
These are divided into two :— 
a. Periodical. b. Not periodical. 
2. Movements independent, at least to some extent, of ex- 
ternal influences, which are also divided into two :— 
a. Periodical. b. Not periodical. 
Fig. 1173. 
Fig. 1173. Nicotiana glauca. A. Shoots with leaves expanded during the 
day. B. The same asleep at night, pointing vertically upwards. (After 
Darwin.) 
(1) MovEMENTS DEPENDING ON EXTERNAL INFLUENCES.—da. 
Peridical.—Under this head we include such movements as 
those of certain leaves and the petals of flowers, which occur 
at particular hours, the organs remaining in the new position 
thus taken up until the return of a particular period, when they 
