THE LEAF 
181 
198. Heliotropism — 
“turning with the sun’”’ —is 
the name given to the daily 
movement of plants like the 
cotton and sunflower in 
turning their leaves or their 
Fics. 235, 236.—Horse-chestnut leaves: 235, leaf rosette seen from above; 
236, the same seen sidewise, showing the formation of rosettes by the lengthening 
of the lower petioles. 
blossoms to face the sun. 
If you live where cotton is grown, 
notice the leaves in a field about ten o’clock on a bright 
sunny morning, and again from the same 
point of view at about four or five in the 
afternoon. Do 
Fic. 237.— Leaf 
oyramid of mullein. 
you perceive any differ- 
ence in their general dis- 
position? Watch on a 
cloudy day and see if 
any change takes place. 
Find out by observation 
whether the “‘ heliotrope”’ 
of the hothouses is really 
heliotropic. 
i9o9. Adjustment 
against too great intensity 
of light.— Plants fre- 
quently have to protect 
themselves against excess 
of light and heat. An 
238 239 
Fias. 238, 239.— A 
compass plant, rosin- 
weed (Silphium lacini- 
atum): 238, seen from 
the east; 239, seen 
from the south. 
