I don't understand what do you mean exactly... But that what I've got...
1. If your question why water comes up in the plant it will be because it goes to leaves of the plant to feed the flower. It moves up by using the stem...
2. But if you mean why gravity force pull water down to the flour and it will come up inside the plant so the answer will be because there are a lot of parts of the plant that needs water.... And actually food that leaves are making for the plant will come to the roots again to make them stronger.