Answer:
Because roots get more water then the stem does, it absorbs at least 60% of the water, and if you use a nail that goes into water, it rusts, so that means that it's darker than others.
Step-by-step explanation:
If you take a nail and place it under water, that means its getting 100% of the water, so it's the same thing with plants. Another reason that this is happening is that its a sign that a plant is unhealthy. So make sure to give it a lot of water( but not too much or it will drown) so it stays a healthy and happy plant. Hope this helps :3