Movement of root is towards the earth and movement of stem is against the earth. A movement towards the earth is negative geotropism. Put a germinating gram seed on soil in a pot and keep the pot in such a closed room so that light cannot come from any direction. After one day it will be seen that the root of germinating gram seed bended and enter into the soil and the stem bended against the soil.
Majority of food for plants remain in earth. Root absorbs these foods. So the movement of root towards earth is quite significant. Different parts of a plant grow in opposite directions because they have different functions. The roots need to grow into the ground to provide stability for the plant, as well as to access water and nutrients.
Hydrotropism: The movement of root is towards water and expansion of stem is against water. The movement of root towards water is positive hydrotropism and expansion of stem against water is negative hydrotropism.
Chemotropism: Earth contains more organic manure thus the root and its branches expanded towards that direction. Movement towards chemical substances is Chemotropism.