sample essays of IELTS

40 Cohesion-tension Theory

Atmospheric pressure can support a column of water up to 10 meters high.
But plants can move water much higher; the sequoia tree can pump water
to its very top more than 100 meters above the ground. Until the end of
the nineteenth century, the movement of water in trees and other tall
plants was a mystery. Some botanists hypothesized that the living cells
of plants acted as pumps. But many experiments demonstrated that the
stems of plants in which all the cells are killed can still move water
to appreciable heights. Other explanations for the movement of water in
plants have been based on root pressure, a push on the water from the
roots at the bottom of the plant. But root pressure is not nearly great
enough to push water to the tops of tall trees. Furthermore, the
conifers, which are among the tallest trees, have unusually low root
pressures.

If water is not pumped to the top of a tall tree, and if it is not
pushed to the top of a tall tree, then we may ask: how does it get
there? According to the currently accepted cohesion-tension theory,
water is pulled there. The pull on a rising column of water in a plant
results from the evaporation of water at the top of the plant. As water
is lost from the surface of the leaves, a negative pressure, or tension,
is created. The evaporated water is replaced by water moving from inside
the plant in unbroken columns that extend from the top of a plant to its
roots. The same forces that create surface tension in any sample of
water are responsible for the maintenance of these unbroken columns of
water. When water is confined in tubes of very small bore, the forces of
cohesion (the attraction between water molecules) are so great that the
strength of a column of water compares with the strength of a steel wire
of the same diameter. This cohesive strength permits columns of water to
be pulled to great heights without being broken.