Growth of Roots, Stems, and Leaves 



23 



the stem of those which are turned away from the lights below 

 the cotyledons, make a row of ink marks. Place in a lighted 

 window. In a day or so the cotyledons will look toward 

 the light. The row of marks will show that the stem has 

 twisted to" bring them into position. When that portion of 

 the stem has stopped its growth, turn the plant halfway 

 around. Will the cotyledons again turn toward the light ? 



Remove the tips from sunflower seedlings, cutting off some 

 above and some below the cotyledons. Do the stems still 

 curve toward the light ? 



Plant sunflower seeds and keep them covered so as to 



Flo. 30. — The phuii in iliis put i^rew on ihe north side of a large rock. The portion 

 beside the pot grew on the south side in the shade. 



exclude all light. At the same time plant others and leave 

 them exposed to light. When the cotyledons of the second 

 lot are well expanded, compare with those which have been 

 covered. The cotyledons are still closely pressed together. 

 Cover those that were left exposed. After a day or two 

 examine them again. Evidently light has an influence in 

 spreading leaves apart ; darkness closes them. 



