Question

How is positive hydrostatic pressure developed in phloem similar to that in a garden hose? And how is negative hydrostatic pressure equivalent to that produced while sucking a straw?

Answer 1

Hydrostatic pressure is the pressure exerted by liquid at rest.Hydrostatic pressure increases with depth because of the increasing weight of fluid exerting downward force from above. This change in hydrostatic pressure with depth is known as hydrostatic pressure gradient . It is responsible for bulk flow in plants.

Positive hydrostatic pressure is responsible for movement through phloem. It works like garden hose , in which also water moves from high pressure site to lower pressure site. In plants, the leaves ( source) have large amount of stored food in the form of sugar . Active transport of solute molecules occurs into sieve-tube element. As a result, water moves from neighbouring cells into the sieve-tube element by osmosis,. This increases hydrostatic pressure , that pushes the sap down the phloem tube.On reaching the sugar sinks, opposite to source site happens.

The negative hydrostatic pressure gradient in roots works similar to how water is sucked in straw. The plant roots absorb water and minerals from the soil. This means that soil has high solute concentration still the water enters into the root hairs along with minerals.