Fact 1: Roots grow in length and width from primary and secondary meristem.

Fact 2: Kids see the underground root systems of trees and then an aerial view of plants in the city.

Given the two facts above, answer the question "What grow in width and length.?" with the following options: 
- crocodiles
 - sedimentary rocks
 - vegetation
 - remains of prehistoric life
 - swamp vegetation
 - shrub beds
 - underground systems
 - loose dirt


 

       underground systems  