For plants, and trees too I guess, you can just grow your own, dig it up after a while, and inspect for yourself.

Today I finished picking tomatoes from my tomato plants and pulled them up to avoid them rotting in the field as the temperature goes down. It was curious to see how the root systems varied both between the two tomato varieties I had planted, the location of the plant in relation to surrounding grass, and the type of soil they ended up in.