15. 
16. 
ROOTS AND THEIR MODIFICATIONS. 47 
The older roots of many plants contract in length. 
Of what advantage can this be? 
Of what advantage are aerial roots ? 
Why do many trees growing in swamps have much 
of their root system near the surface ? 
What is the origin and purpose of the root tubercles 
of some plants ? 
How are some plants propagated by their roots, e.g. 
sweet potato and locust tree ? 
How do gravity, moisture, and light affect the 
growth and direction of roots ? 
Experiments 21 and 23. 
Of what economic importance are roots? 
What is the purpose of “root pressure,” and is it 
always apparent? Why? 
Experiment 16. 
