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 

 paratonic 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." InMimosa the leaflets are carried 



(Drosera rotundifolia) with half b the m()tor Q f orwa rd and Upward 



of the tentacles innexed from J 



stimulation. Adapted from until the upper faces are pressed together, 



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 



