IRRITABILITY. 



'33 



273. Transmission of the stimulus. The transmission of 

 the stimulus in this mimosa from one part of the plant has been 

 found to be along the cells of the bast. 



274. Cause of the movement. The movement is caused by 

 a sudden loss of turgidity on the part of the cells in one portion 

 of the pulvinus, as the cushion is called. In the case of the 

 large pulvinus at the base of the petiole this loss of turgidity is 

 in the cells of the lower surface. There is a sudden change in 

 the condition of the protoplasm of the cells here so that they 

 lose a large part of their water. This can be seen if with a sharp 

 knife we cut off the petiole just above the pulvinus before move- 

 ment takes place. A drop of liquid exudes from the cells of the 

 lower side. 



275. Paraheliotropism of the leaves of the sensitive plant. If the mimosa 

 plant is placed in very intense light the leaflets will turn their edges toward 

 the incidence of the rays of light. This is also true of other plants in 

 intense light, and is paraheliotropism. Transpiration is thus lessened, and 

 chlorophyll is protected from too intense light. 



We thus see that variations in the intensity of light have an important 

 influence in modifying movements. Variations in temperature also exert 



a considerable influence, rapid 

 elevation of temperature causing 

 certain flowers to open, and 

 falling temperature causing 

 them to close. 



276. Sensitiveness of insec- 

 tivorous plants. The Venus 

 fly-trap (Dionaea muscipula)and 

 the sundew (drosera) are in- 

 teresting examples of sensitive 

 plants, since the leaves close in 

 response to the stimulus from 

 insects. 



Fig. 126. 



Leaf of Venus fly- 

 trap (Dionaea musci- 

 pula), showing winged 

 petiole and toothed 

 lobes. 



Fig. 137. 

 Leaf of Drosera ro- 

 tundifolia, some of the 

 glandular hairs folding 

 inward as a result of a 

 stimulus. 



277. Hydrotropism 



Roots are sensitive to mois- 

 ture. They will turn toward moisture. This is of the greatest 

 importance for the well-being of the plant, since the roots will seek 

 those places in the soil where suitable moisture is present. On 



