MIMOSA 



51 



The continuous observation of half a dozen 

 healthy plants through the course of a mid- 

 summer's day will reveal the greater number 

 of reactions exhibited hy this plant. 



Early in the morning the pinnules are seen 

 to occupy a horizontal position with the 

 blades fully exposed to the light. The petioles 

 are slightly elevated above the horizontal. 

 As the sun mounts toward the noonday posi- 

 tion its rays increase in intensit}^ and their ef- 

 fect on horizontal leaf-blades will increase cor- 

 respondingly. At sometime, however, before 

 the rays strike the surface perpendicularly, the 

 regulatory mechanism of the plant sets up 

 movements in the pinnules by which their 

 surfaces are directed upward at an acute 

 angle. The angle increases as the sun nears 

 and passes the zenith until the edges of the 

 blades are directed almost exactly toward 

 the sun, and its rays exercise an actual effect 

 on the leaf not much greater than in the 

 early forenoon. If this adaptation were not 

 made, the fierce rays would strike through 

 the leaf-blades so strongly as to injure the 

 chlorophyll and evaporate more water than 

 could be supplied by the roots, thus causing 

 wilting. 



As the sun declines toward the west, the 

 blades return once more to the horizontal 

 position, but the approach of night brings 



Day positions 

 of leaves 



Night positions 

 of leaves 



