454 
PHYSIOLOGY 
Under unfavorable conditions they cease, but the plant may still be 
able to respond to external stimuli like others about to be described. 
Paratonic movements. The terminal leaflet of Desmodium gyrans, 
like leaves of other members of the 'bean family, exhibits par atonic 
movements (i.e. those due to special stimuli, not tonic; opposed to auto- 
nomic). Moreover, some plants whose leaflets ordinarily exhibit only 
paratonic movements, may make autonomic ones under exceptionally 
favorable conditions. Thus it would seem that there is no fundamental 
difference in the two, and when the precise stimuli that initiate the move- 
ment are discovered, autonomic movements may all be relegated to the 
par atonic category. 
Turgor movements due to external stimuli are numerous and easily 
observed, but except in a few striking cases they are not rapid enough 
to be seen by watching for a brief time. The stimuli initiating the move- 
ments are of the most varied character; contact, gravity, and changes 
of light and temperature being the most common. 
Contact movements. If the stamens of the barberry (Berberis) be 
touched near the base at the time when they are shedding pollen, they 
suddenly fly up and inward, carrying the 
anthers close to the stigma. After a short 
time they resume their former position against 
the petals. The filaments of the Cynareae, 
a tribe of Compositae, shorten instantly on 
being touched (the reaction time is less than 
i sec.), dragging the coherent anthers quickly 
down over the style, whose hairs scrape out 
the pollen like a pipe cleaner. In Centaurea 
americana, this contraction continues for 
7-13 seconds, and after a minute the rest 
position is again reached. 
Probably the best known of the rapid 
contact movements are those of the species 
of Mimosa and Biophytum, the " sensitive 
FIG. 685. Leaf of sundew plants." In Mimosa the leaflets are carried 
(Drosera rotundifo/ia) with half b the motor Q g f orward and upwar d 
of the tentacles inflexed from ' 
stimulation. Adapted from until the upper faces are pressed together, 
KERNER. while the primary motor organ drops the 
whole leaf (fig. 683, p. 452). Another famous example is the quick 
closure of the " fly-trap " of Dionaea (figs. 657, 658, p. 386). Here 
