Question

Using very precise measurement you can show that the diameter of tree trunks fluctuates throughout the day with the trunk being widest in the early morning and shrinking throughout the day. This decrease in diameter is apparent in the upper part of the trunk before it starts in the lower part. These two observations have been used to support the hypothesis that transpirational pull is the mechanism of water movement in through the tree.

Explain how these observations of the fluctuation in diameter supports this hypothesis.

Answer 1

When the upper parts of the tree loose water by the process of transpiration. This loss occurs more from the leaves and young parts of the tree.

This loss in water causes the plant cells to shrink a little only as due to presence of cell wall they cannot collapse. Because of this shrinkage the diameter of the upper part of the tree trunk decreases. More over this is clear indication that negative pressure must would have developed in the upper part of the tree . This negative pressure in the upper part of the tree pulls water from the lower part of the tree trunk due to which with the passage of time the lower part also decreases in diameter. This clearly explains that the transpiration develops a negative pressure in the upper parts of the tree and this negative pressure pulls the water to the top of the tree. Thus, transpirational pull hypothesis explains the mechanism of movement of water yo the top of the tree.