However, in all land plants, gravity is significantly pushing down on the plant which means that, if the plant is trying to grow vertically, it must have strong structural components to support it, the bottom needing stronger than the top. This is easily seen in trees which taper as one follows them up. In water, this is different as the buoyancy of plants (or algae, which aren't quite plants except for maybe green algae), is very similar to that of water (almost the same density) and plants can grow low density parts such as little pockets of gas which help to keep the plant afloat.