Water chemically bonds with soil particles to form a film around the soil particles. As more and more water rains down upon the soil it percolates deeper into the ground. As the water in the surface soil is depleted the roots need to grow deeper to get to moist soil. This is desirable because deep roots help plants better survive draughts and strong winds. Through a process called osmosis, water enters the plant through its' root system.
Osmosis is the equalization of water, salt, sugar and mineral concentrations.
The concentration of salts, minerals and sugars is typically higher in plant cells than it is in the soil. In an attempt to dilute the concentration of these substances in the plant cells water travels out of the soil and into the plant cells. While cell walls allow water to enter the cells they do not allow salts, minerals and sugars to leave the cell. In this manner plant cells gain the water they need and do not loose nutrients. Parts of the plant's roots called the epidermis and endodermis act like pumps, pushing the water up the plant through vein like structures called xylem.