If you grind up a human body, bones and all, and make a chemical analysis of what the body is made of, you'll find that in terms of weight we're mostly water -- sixty to eighty percent of every human body is nothing but water. The same is true of plants. About seventy-five percent of a typical land plant is water.
These high water contents reflect the evolutionary histories of both plants and animals: The ancestors of both came from the seas. Even today when humans cry or sweat it's saltwater that comes out. Humans can go for weeks without eating, but only for a few days without drinking.
Therefore, the topic of roots is an important one, for roots are the means by which most land plants acquire their water. Roots grow through the soil, soak up what water they can, and transport that water to the plant's other parts.
ROOTS AREN'T STRAWS
Here's something interesting to think about: Have you ever noticed that when you dig into soil, unless there's been a recent rain, precious little water appears to be present? Obviously, plant roots, in order to find so much water in a place where there doesn't seem to be much of it, must be doing something special.
When a typical plant sends down roots, it's not at all like digging a well down to the water table. Typical roots don't grow downward through the soil until they hit water so they can suck up the water as if the roots were straws. Roots work in a whole different way, a way based on the principle of osmotic pressure, a way that enables plants to acquire water from soil that may look and feel fairly dry. You should find out more about osmotic pressure at school or on the Internet, since here we're just interested in the roots themselves.
When a seed germinates it sends up a sprout, and it sends down its first root, a special root referred to as the radicle. Soon the radicle sprouts many, perhaps thousands, of very slender, thinner-than-a-hair root hairs, which give the radicle a fuzzy-white appearance.
The fuzzy stuff on the two much-magnified roots at the right are root hairs on a grass root. Each root hair in this picture is less than 0.04-inch long (1 mm). Root hairs also form at the tips of much larger roots as they grow through the soil.
One interesting thing about root hairs is that each hair is essentially a very long, slender bulge from the side of a single cell embedded in the "skin" of the root it's growing from. In other words, every root hair along with its foot in the main root's "skin" is just one cell that you can actually see with your naked eyes. Of course, most cells are so tiny that they're practically invisible.
When you pull up a plant and look at its long, beard-like cluster of blackish roots, very little of what you're seeing actually absorbs water for the plant's use! The big roots you're seeing help anchor the plant in the ground, and they do transport water, but nearly all the plant's water is absorbed not by them, but by the root hairs.
In a classic study by H.J. Dittmer, a single rye plant -- a kind of grass -- growing about 20 inches high and consisting of a clump of about 80 shoots, was found to have 380 miles of roots, and this included 14 billion root hairs! If all these root hairs had been split open and spread flat on a floor, their combined surface area would have covered more than 4,000 square feet -- about the floor space occupied by two or three good-sized houses. All this root-hair surface-area was absorbing water for that single clump of grass... No wonder rye can grow in fairly dry soil!
In typical soil, water is available to plants mostly as a thin film surrounding every microscopic soil particle. Root hairs absorb this water when they grow into the soil-particle's vicinity and then, molecule by molecule, water diffuses through the root-hair's cell membrane, flows toward the main roots, and then enters a kind of pipeline carrying the water upward to the rest of the plant.