why water would not pass up a tree if it has very little cohesion
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Water is often the most limiting factor to plant growth. Therefore, plants have developed an effective system to absorb, translocate, store and utilize water. To understand water transport in plants, one first needs to understand the plants' plumbing. Plants contain a vast network of conduits, which consists of xylem and phloem tissues. This pathway of water and nutrient transport can be compared with the vascular system that transports blood throughout the human body. Like the vascular system in people, the xylem and phloem tissues extend throughout the plant. These conducting tissues start in the roots and transect up through the trunks of trees, branching off into the branches and then branching even further into every leaf.
"The phloem tissue is made of living elongated cells that are connected to one another. Phloem tissue is responsible for translocating nutrients and sugars (carbohydrates), which are produced by the leaves, to areas of the plant that are metabolically active (requiring sugars for energy and growth). The xylem is also composed of elongated cells. Once the cells are formed, they die. But the cell walls still remain intact, and serve as an excellent pipeline to transport water from the roots to the leaves. A single tree will have many xylem tissues, or elements, extending up through the tree. Each typical xylem vessel may only be several microns in diameter.
"The physiology of water uptake and transport is not so complex either. The main driving force of water uptake and transport into a plant is transpiration of water from leaves. Transpiration is the process of water evaporation through specialized openings in the leaves, called stomates. The evaporation creates a negative water vapor pressure develops in the surrounding cells of the leaf. Once this happens, water is pulled into the leaf from the vascular tissue, the xylem, to replace the water that has transpired from the leaf. This pulling of water, or tension, that occurs in the xylem of the leaf, will extend all the way down through the rest of the xylem column of the tree and into the xylem of the roots due to the cohesive forces holding together the water molecules along the sides of the xylem tubing. (Remember, the xylem is a continuous water column that extends from the leaf to the roots.) Finally, the negative water pressure that occurs in the roots will result in an increase of water uptake from the soil