John and emily were experimenting with the effects of road salt on plant growth. to do this, they watered plants daily with the same amount of salt water. they could best improve their experiment by -

They could improve the experiment that they made by adding another variable such as: watering the plants with fresh water. This way they could have a better comparison how plants grow based on the type of water they are exposed with.  

I’m not exactly sure what your trying to ask, but if it’s how an organelle is similar to a body system or a part a nucleus is like a in function. but i don’t fully understand what you’re trying to ask