owever, 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.
Gravity is also used by at least some plants to determine which direction to send above ground growth and which to send below ground growth. See also: Does gravity affect plant growth.
Gravity helps in other ways as well, providing a maintenance function, pulling detritus, water, dust, and most other things down towards the ground, keep the upper parts of plants relatively clean. This becomes apparent if one tries to grow plants in zero gravity. Even if one made the dirt in such a way that it will not fly off, dead plant parts would accumulate in the air, slowly blocking out more light unless steps are taken to prevent this.
Other design elements of plants depend on the force of gravity. Leaves, for example, if suddenly exposed to zero gravity may not be in the same place as they were in gravity.
There may also be many other processes in plants which depend on gravity for their function. Some plants have been grown in zero gravity or micro gravity conditions, so despite all the disadvantages, at least some plants can still grow.
Ultimately, gravity limits the size and height of plants in that they cannot grow beyond a certain height and size due to their physical structure not being able to support the mass of the plant against gravity's effects.