Question

Explain why xylem transport is unidirectional and phloem transport bidirectional.

Answer 1

Xylem transport is unidirectional because the water column flows only upwards due to two major forces i.e. the transpiration pull and the positive root pressure. Transpiration causes a negative pressure whereas root pressure is positive which sends the water upwards towards the shoot region from the roots.

Phloem transport is bidirectional because sucrose gets accumulated in the phloem tissue and absorbs water creating a high turgor pressure. Other areas have low turgor pressure due to loss of water. Hence the water flows from regions of high turgor pressure to low turgor pressure and the direction can be upwards or downwards from the leaves based on the gradient.