Gravitational load and defying gravity

Buoyancy

Although the gravitational 'force' on the surface of the Earth is more or less constant, the mechanical load on land-life is around 1,000 times greater than in the water. Underwater, gravity is almost cancelled-out by buoyancy

...the upward force exerted by a fluid that opposes the weight of an immersed object. [1]

If a body is lighter (less dense) than water, it will float. It floats as the body is experiencing an upward force that is equal to the weight of an equivalent volume of water, minus the weight of the body. If a body is denser than water, it will sink.

Different rules in the sea - defying gravity

All sea animals are mostly made up of salt water (relative to their total mass), so they weigh very close to the same as an equivalent volume of water. This means that their apparent weight (mechanical load) is very close to zero. And so, although, life in the sea has to deal with other forces, such as high levels of drag and lift, it is this difference in mechanical load that has made it possible for the largest mammal (and animal) on Earth to evolve in the sea - the Blue Whale - and not on the land; and in essence, defy gravity. As a side note, Whales are mammals, meaning they need to breath air from the atmosphere, so they have adaptions, such as extremely high levels of haemoglobin in the blood and muscle cells, that means that when they deep dive, there is actually very little air in their lungs (which get compressed to a very small size, as the increasing weight of the water above presses down on them), and they are able to 'breath' from the oxygen resources in the blood cells and muscles and not from the compressed lungs.

Seaweed - a brown, macro-algae, such as kelp, develop into huge forests under the sea. These ecosystems are recognised as some of the most productive and dynamic ecosystems on Earth. Like the whale, the seaweed does not have to 'deal' with gravity, and so unlike land plants, seaweed doesn't have to expend energy building complex support materials, such as lignin; and as it obtains all it's nutrients from the sea-water that is all encompassing, it doesn't need complex vascular (xylem and phloem) systems to transfer nutrients around, or roots - instead they have a short root-like mass, known as a holdfast for anchoring the thallus (the body) on to the sea floor. For these reasons, some seaweeds can grow faster than tropical bamboo (about 30cm per day for some species), and as such, can drawdown far more carbon dioxide from the atmosphere than land plants.

Life on Land - defying gravity, in some ways, and only up to a point...

Life on land, on the other hand, has had to evolve strategies to overcome (or at least mitigate) gravity, with one of the greatest feats accomplished by plants - the tallest organism on land - with their ability to transport water up and through their structures against the forces of gravity - 'upwards' - without expending any energy (at least not directly).

The largest tree on Earth is the Sequoia of California (coastal Redwoods), which stand at around 150 metres tall, with a volume of about 1500m3. However, it is gravity (and electromagnetism) that limits the height of the tallest trees.

The electromagnetism defines the strength of the chemical bonds in the lignin (the hard, woody material that makes up much of the tree trunks and branches), and so the trees' ability to support its' own weight is defined by the electromagnetic forces in lignin. And plants, unlike seaweed, also need to pump water from the ground to all their leaves, and up to the very top leaf, transporting nutrients and water upwards, thanks to a combination of different physical properties of water.

One property in particular, is that when water evaporates out of the pores of the leaves (thanks to the heat from the Sun - therefore solar energy), and as the different water molecules are held together by hydrogen bonds, the rising water molecule that is released into the atmosphere, pulls the water molecule below upwards. The lower molecule does the same, to the one below it, and this pulls water up the plant like a daisy chain up the xylem tubes (the water capillary tubes). A tree of this size can raise upto 4 tonnes of water every day. However, this is also limited by electromagnetism, as the hydrogen bond daisy chain, can only pull a limited weight (which is the mass of water x gravity), before it breaks; and this is calculated to be the height of the giant Redwoods. To underline the point, as the gravitational force on Mars is a lot weaker than on Earth (about 38%) for instance, in theory, the same tree could grow taller - up to 200 meters tall on Mars, as the electromagnetic force would be the same...

There is also a hypothesis that tree snakes, that are most often in the vertical position, are effected more by the direction of the gravitational force, which has a orientational effect on the flow of blood, so that the heart has evolved closer to the brain, compared with their more horizontally oriented land and water relatives [Harvey B. Lillywhite]. 

Sensing Gravity

Many forms of sea-life, and land-life have also evolved the ability to sense gravity. Specific cells within many different organisms have evolved specialised structures, to sense and convert gravitational forces into biological signals. For example, otoliths in hair cells of the inner ear of many vertebrates help balance (and depth detection for fish), and statoliths in plants for sensing up and down - helping plants determine the direction of growth (upwards for stems to reach more light, and downwards for roots to reach more nutrients and water...).