Given a scientific question, generate a correct answer to it.

Example Input: What kind of energy is specifically defined as the energy that is available to do work?
Example Output: free

Example Input: What done ozone help protect against?
Example Output: ultraviolet rays

Example Input: The stems of all vascular plants get longer through primary growth. this occurs in primary meristem at the tips and ______ of the stems.
Example Output:
nodes