112 ELEMENTS 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 XII 
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. au 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 heliotia- 
pism. In both eases the movement is produced by unequal 
erowth brought about by the unequal lighting of different 
sides of the stem. If the less strongly Lehted 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. 
; for vertical twigs. 
Leaf arrangement oa Se 
i for horizontal twigs. 
Movements of leaves. . . Uses of. 
Compass-plants. 
{ positive. 
} 
Hehotropic bending of stems . é 
h negative. 
