GROWTH OF ROOT AND STEM 29 
taken place. It has not taken place at the tip, for the 
mark made near the tip is still there. The curve has 
taken place back from the tip, in the region of mark 38, 
4, or 5, probably, if the marks were close together at 
first. These marks on the bent region of the root are 
now far apart. 
You remember that when the root was measured to 
see where growth in length took place, we found that 
the root grew in this 
same region, just back 
of the tip. This is an 
interesting observation, 
and I think you can un- 
derstand why the root 
can bend easier in the 

region where it 1s 
stretching than in the ie 54. Bean seedling placed horizontally and 
region where elonga- marked to show where the root bends. 
tion has ceased. The region of elongation 1s called 
the motor zone, because this is where the root moves. 
What causes the root to turn downward? ‘This isa 
question that is difficult, perhaps, to demonstrate to 
your satisfaction. It can be shown, however, that 
gravity influences the root to turn toward the earth. 
Gravity, you know, is the force which pulls an apple 
or a stone toward the earth when either is let fall. 
We must bear in mind, however, that gravity does not 
