Most plants and trees get the water and minerals they need to survive from their roots. Basically, water is absorbed from the soil then travels up through the plant roots to the stems and then to the leaves. Plants have to create pressure in the system to move water from the roots up to the leaves and this is difficult because plants have no heart or internal pump to move the water. So, you can imagine it must take lots of effort for water to rise to the top of a plant, let alone to the top of a tree that is 300 or more feet tall. In fact, plants and trees actually acquire water by osmosis through root hairs, water is then transported or pulled upward by a process called transpiration. The pulling force of transpiration involves water potential, energy of evaporation, surface tension and hydrogen bonds. (Solomon et al 2002). .
Water mainly enters a plant through tiny root hairs growing on the root. Water and other dissolved minerals then move from the root tissues to the vascular cylinder. Plants have two main types of vascular tissue that transport water, called xylem and phloem. Xylem is the vascular tissue that transports water and minerals from roots to leaves. Phloem can transport water both upward and downward. (Solomon et al 2002). .
Originally, water enters the xylem in the roots of plants and trees by Osmosis. Osmosis is the diffusion of water through cell membranes. Once in the xylem, root pressure results from root cells pushing xylem sap upwards. Root pressure may push the water up a bit, but this still doesn't account for water movement to the top of a tall tree. .
Less than one percent of the water in a tree that actually reaches the leaves is used in photosynthesis and plant growth because most of it is lost in a process called transpiration. Transpiration provides two basic functions: it provides the force for lifting the water up the stems and it cools the leaves. Transpiration is the evaporation of water from plants.