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 
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 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 
