400 



RELATION TO ENVIRONMENT. 



ulus were entirely removed. As we know also contact or pres- 

 sure stimulus, or jarring produces the same result in " sensitive" 

 plants like mimosa, some species of rubus, etc. In many plants 

 there is no well-developed pulvinus, and yet the leaves show 

 similar movements in assuming the day and night positions. 

 Examples are seen in the sunflower, and in the cotyledons of 

 many plants. A little observation will enable any one interested 

 to discover some of these plants.* In these cases the night 

 position is due to epinastic growth, and while this influence is 

 not removed during the day the light stimulus overcomes it 

 and the leaf is raised to the day position. 



770. Leaves which rotate with the sun, During the growth 

 period the leaves of the sunflower as well as the growing end 



Fig. 439- 

 Same sunflower plant photographed just at sundown. 



of the stem respond readily to the direct sunlight. The re- 

 sponse is so complete that during sunny days the leaves toward 

 the growing end of the stem are drawn close together in the 

 form of a rosette and the entire rosette as well as the end of the 



* Seedlings are usually very sensitive to light and are good objects to 

 study. 



