184 OUTLINES OP PLANT LIFE. 



to a touch, shortening, elongating, or bending in such a way 

 as to promote pollination ( ^ 295). 



The motor organs of the leaves of a number of the bean and 

 oxalis families also react to more violent mechanical stimuli. 

 Their movements are similar to those described in ^ 255. 



257. Summary. — By irritability, that is, the sensitiveness 

 of protoplasm to external agents, plants are able to regulate all 

 their activity and adjust themselves to the world about them. 

 Under unfavorable conditions this sensitiveness is temporarily 

 lost. If permanently lost, it is death. It is more marked in 

 some parts than others and its effects in these parts are capable 

 of being transmitted to distant parts. 



The reactions of plants to stimuli are most easily observed 

 when they result in movements. Movements of the proto- 

 plasm itself seem to be automatic, but can be directed by ex- 

 ternal stimuli. Movements of multicellular plants are due 

 either to unequal growth or to unequal turgor. Light, 

 heat, gravity, moisture, or contact may so influence the rate 

 of growth, or the amount of turgor as to cause curvature of 

 growing parts or of a special motor organ. The parts affected 

 may thus be turned toward or away from the source of the 

 stimulus, or may be placed transverse to it. Movements in 

 response to gravity, light, and heat are most important. 

 These work conjointly to determine the position of organs. 



