Animals get most of their energy by breaking down carbohydrates and fat. They acquire this energy rich provender by either eating other animals, or by eating plants - or both, as human beings do. Organisms like us, which need their food ready-made, are called 'heterotrophs'. But the buck has to stop somewhere - and in most earthly ecosystems, it stops with plants. Plants make their own carbohydrates, fats, proteins, and everything else they need from raw materials - simple chemical elements, and the simplest possible chemical compounds. They obtain the energy to do this from the sun. They are 'autotrophs': self-feeders.
They key to autotrophy is photosynthesis. Within their leaves plants harbour the wondrous green pigment known as chlorophyll. Chlorophyll traps units of energy - photons - from sunlight. Then, acting as a catalyst, it uses the photon energy to split molecules of water. Where there was H₂O, now there is H plus O. The O - oxygen - floats away into the atmosphere as oxygen gas. If it weren't for photosynthesis, there would be no oxygen gas at all in the atmosphere, and creatures like us could never have evolved at all.
The interesting bit in this context is the hydrogen, which is then combined, within the leaf, with carbon dioxide gas from the atmosphere. Thus simple organic acids are created, compounded from carbon, hydrogen and oxygen. These simple compounds, with a little more manoeuvring, are transformed into sugars (the simplest carbohydrates). When the sugars are modified a little more, they become fats. Add nitrogen, and they can be made into proteins. Incorporate a few other chemical elements, and all the components of living tissue can be made. Chlorophyll itself is basically a protein, with some magnesium at its centre.
Green plants are engines of photosynthesis. it is what they do, their raison d`être, and we should be properly grateful that it is, for without their ingenuity and labour, insouciant heterotrophs like us could not exist. Trees are the greatest of nature's engines of photosynthesis. Their need to photosynthesize explains the whole ,vast, elaborate architecture of the tree. Leaves are the meeting place of carbon dioxide (wafting in from the air), water (drawn up from the ground) and sunlight. All are brought together in the presence of chlorophyll, which acts as host and mediator.
Leaves archetypically are flat and thin, to expose the chlorophyll within them to as much sunlight as possible. The chlorophyll is held in loosely-bound cells in the middle layers of the leaf - a spongy arrangement, so air can circulate freely. The air enters through perforations underneath the leaf, known as 'stomata', which open and close according to conditions (generally closing when it is too dry, and the leaf is in danger of wilting, and also, typically, when it is dark). All green plants do this - but trees, the greatest of plants, hold their leaves as high in the sky as possible, for maximum exposure to air and sun. The water (and minerals) come mainly from the ground - sometimes from deep below the ground - and so must be carried upwards through all the length of the roots and trunk and branches to the leaves aloft.
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The whole vast and intricate structure is evolved to bring air and water together in the presence of sunlight; and the water and attendant minerals come mainly from the earth.
But how can a tree take water to such heights?
Some plants derive their water from the air. Some trees do this too: extraordinarily, the mighty redwoods of California get about a third of their water from the morning fogs that sweep in from the Pacific. Mostly, however, trees draw water up from the ground, through the conducting vessels of the xylem, coursing through their trunks and branches. It would be wonderful with X-ray eyes to see a forest without the timber. It would be a colony of ghosts, each tree a spectral sheath of rising water.
But how does the water rise up to the leaves?
Now it seems that water is sucked up from above by a combination of osmosis and evaporation. The sap in the cells of the leaf interior is a concentrated solution of minerals and organic materials, and water from the conducting vessels flows into them by osmosis. Because the cells of the leaf interior are open to the air (via the stomata), the water within them evaporates and exits via the stomata (and to some extent through the leaf surface in general). As water is lost, so the sap that remains in the leaf cells becomes more concentrated - and so more water is drawn from below.
So water is dragged from above by the leaves, up through the vessels of the xylem, not in a crude and turbulent gush but in millions on millions of orderly threads. Each liquid thread is only as thick as the bore of the conducting vessels: the biggest are 400 microns across (0.4 of a millimetre) and most are far smaller than this. The tension within them is enormous: the threads are taut as piano wires. Water molecules cling tightly together. Their cohesive strength is prodigious. Were it not so, trees could not pull water from below, and could not grow so tall; but in practice the forces are such that a tree could grow to a height of three kilometres if the tensile strength of water was the only constraint on its growth.
The final evaporation of water from the leaves, out through the stomata, should perhaps be seen as a side effect of the whole mechanism. .. it seems that most plants (including all trees) lose water through the stomata simply because this is very difficult to avoid; or at least, the loss is a price worth paying to maximise the efficiency of photosynthesis. The point of the plant's architecture - all those conducting vessels, all those perforated leaves - is to bring the Greek elements together: to present water to the sun, in the presence of air. But it is hard bring them together without losing water, and sometimes losing more than the plant would like.
The overall effect is a flow of water from the roots, through the vessels to the leaves and out to the atmosphere: trees act like giant wicks. The final loss of water by evaporation is called 'transpiration'; and the total flow of water from soil to atmosphere is the 'transpiration stream'. The overall magnitude of this stream, especially when several trees are gathered together, can be prodigious; and its effect on soil and climate, and thus on surrounding vegetation and landscape, is critical to all life on earth, including ours.
from "The Secret Life of Trees" - Colin Tudge
Now you see how trees lose moisture to the skies, which of course is how clouds are created (other than evaporation from bodies of water), there is a method the trees use to try and get that water *back* from the skies..
"For a raindrop to form the water needs a nucleus around which it can take shape. This can be dust from the atmosphere or a particle of sulphur from the ocean. But a scientific breakthrough has shown raindrops can form around a nucleus of bacteria.
These bacteria are released in massive quantities by the rainforest. Broad leafed rainforest plants release billions of tons of these aerobacter into our atmosphere. These aerobacter seed the clouds. And it's this seeding process that creates much of the world's rainfall.
But the rainforests do more than provide us with rain. The huge amount of water vapor transpiring from the abundance of leaves creates a vast cloud canopy giving us shade and cooling some of the hottest parts of the world.This cloud cover also reflects much of the sun's heat back out into space, without it life on earth would be unbearable.
The rainforests work in unison with the ocean and the air currents. Together they function as a grand global air conditioning system.
Nature's way of regulating the world's climate."
from David Warth's production "Rainforest: The Secret of Life", narrated by Jack Thompson
