Hint: Stem and roots make the two most essential parts of a plant. Both of them have specific roles to play in plant growth and development. The roots are the underground structure that usually lies at the base of the stem of the plants.
Complete answer: Roots are thread-like fibrous structures that lie in the soil and they usually originate at the base of the plants. The roots grow and elongate inside the soil. Their main roles are to provide anchorage and support to plants. The roots have absorbing root hairs that help to suck up the water and minerals from the soil. Water and minerals are then transported to the rest of the plant parts through transpiration and other water pulling mechanisms. Thus roots provide water and minerals to the plants and secondly they provide support to plants. All of the terrestrial plants have dense root networks that help them to stay rooted in the soil and stand upright. Now, aquatic plants live in water bodies and thus they do not require roots for the purpose of water absorption. But aquatic plants do require roots for anchorage. The floating aquatic plants have very few roots which are very thin like in Pistia. Deep submerged plants like Nymphea have developed strong roots that help to keep them anchored to the soil in water. Thus, the difference in aquatic roots is that they are reduced and thin as compared to terrestrial plant roots.
Note: Terrestrial and aquatic plants have various differences as they have very different surroundings. Terrestrial plants develop a mechanism to reduce water loss. But aquatic plants have no problem with water loss and thus they stress more on buoyancy and support.