Push/pull theory


Plants need much more water to survive than animals do, a sunflower must take in 17 times more water than a human of comparable weight. At least 500 pounds of water must be lifted from the roots to the leaves to make a single pound of organic tissue. This tremendous amount of water is required to undergo photosynthesis each and every day of a plant's life. But how does a tree lift hundreds of gallons of water hundreds of feet in the air against the force of gravity? Scientists have developed two theories to explain this "impossible" phenomenon. In the following, I will discuss the theories and show how the pull theory is more viable.

The push theory states that positive pressure pushes up from the roots continuously and that force maintains a constant flow of water circulating throughout the plant. This constant pressure that exerts force from the water it takes from the ground, supposedly should provide enough power to continue the circulation and distribution of water efficiently throughout the entire plant. Proof of root pressure is exhibited through a process called guttation. Through guttation, tiny droplets of water are sometimes forced out of some plants. Guttation only works when soil is almost completely saturated with water and leaves are not losing much water through evaporation. When the xylem is full of water, the molecules will not move back down the structure, but rather need to be expelled from the plant. They will move, by the process of osmosis, to an areas where there is the least amount of pressure. These areas are usually openings located on plant leaves. Once the water molecules reach the leaves, the excess water will be forced out of the plant by root pressure.

This push theory presents two major inconsistencies 1)many plants exhibit no root pressure and 2)root pressure is not strong enough. In many plants and trees, there is a substantial amount of root pressure that could possibly explain the movement of so much water throughout a tree, defying gravity. However, in many plants, root pressure is not nearly enough to exert the amount of force needed to circulate such large quantities of water throughout the plant and against gravity. There is also absolutely no root pressure in many trees and shrubs. When a plant stem is punctured, water does not spew out, but rather, air is sucked in. So how can this theory be correct?

The pull theory was developed in 1895 by Irish plant physiologists H. H. Dixon and J. Joly. They proposed that water was being pulled up the plant by tension, or negative pressure, from above. ...