112 ESSENTIALS OF BOTANY 



above ground usually bends toward the quarter from which 

 the strongest light comes. Such movements are called 

 heliotropic from two Greek words which mean turning 

 toward the sun. How do the plants in a window behave 

 with reference to the light ? 



EXPERIMENT XIII 



How do Young Shoots of English Ivy bend with Reference to Light? 



— Place a thrifty potted plant of English ivy before a small window, 

 e.g., an ordinary cellar window, or in a large covered box painted 

 dull black within and open only on the side toward a south window. 

 After some weeks note the position of the tips of the shoots. 

 Explain the use of their movements to the plant. 



130. Positive and Negative Heliotropic Movements; how 

 produced. — Plants may bend either toward or away from 

 the strongest light. In the former case they are said to 

 show positive heliotropism, in the latter negative heliotro- 

 pism. In both cases the movement is produced by unequal 

 growth brought about by the unequal lighting of different 

 sides of the stem. If the less strongly lighted side grows 

 faster, what kind of heliotropism results ? If the more 

 strongly lighted side grows faster, what kind of heliotro- 

 pism results? How would a plant behave if placed on a 

 revolving table before a window and slowly turned during 

 the hours of daylight ? 



131. Review Summary of Chapter XI. 



Leaf arrangement . . 



J for vertical twigs. 

 I for horizontal twigs. 

 Movements of leaves . . . . ' Uses of. 

 Compass-plants. 



Heliotropic bending of stems . J ^°^^ '7^" 



,| negative. 



