148 FOUNDATIONS OF BOTANY 



day, strike the leaf-surfaces nearly at right angles during 

 a considerable portion of the forenoon and afternoon, 

 while at midday only the edge of each leaf is exposed 

 to the sun. 



156. Movements of Leaves and Stems toward or away 

 from Light (Heliotropic Movements). The student doubt- 

 less learned from his experiments with seedling plants 

 that their stems tend to seek light. The whole plant 

 above ground usually bends toward the quarter from which 

 the strongest light comes. Such movements are called 

 Tieliotropic from two Greek words which mean turning 

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

 with reference to the light ? 



EXPERIMENT XXVII 



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. 



157. 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 



