Transport in Plants

The transport system in plants involves two types of vascular tissues, xylem and phloem.
The xylem transports water and dissolved minerals from roots to the rest of the plant.
Water movement in the xylem is explained by the cohesion-tension theory, which states that water molecules stick together (cohesion) and are pulled up the plant due to the evaporation of water from the leaves (transpiration).
Phloem tissues are involved in the transport of food in the form of sugars (translocation) from leaves (source) to the rest of the plant (sink).
The movement of food in the phloem is explained by the pressure-flow hypothesis or mass flow theory, which states that sugar is actively loaded into the phloem, causing water to follow by osmosis and create a high pressure, driving the flow of sugar toward areas of lower pressure.

Role of Root Hair Cells

Root hair cells increase the surface area for absorption of water and mineral ions by the plant.
Water is absorbed by osmosis while ions are taken up by active transport.
The high concentration of ions inside the root hair cells creates a water potential gradient, causing water to be drawn into the cell by osmosis.

Transpiration is the process of water movement through the plant and its evaporation from aerial parts, largely from leaves but also from stems and flowers.
It creates a transpiration pull that helps in drawing water up the plant from roots in the xylem vessels.
The rate of transpiration can be affected by factors like light intensity, temperature, air flow, and humidity.
To prevent excessive water loss, plants have stomata, small openings in leaves which can open and close to control the rate of transpiration.
The opening and closing of stomata is controlled by guard cells, which respond to changes in light and CO2 levels. When guard cells are turgid, stomata open; when they lose water and become flaccid, stomata close.

Desert plants, or xerophytes, have adaptations such as thick waxy cuticles, rolled leaves or reduced leaf size to reduce the surface area for water loss.
Some desert plants have sunken stomata to trap moist air and reduce transpiration.
They also have special adaptations called crassulacean acid metabolism (CAM), involving the uptake of CO2 during the night to reduce water loss during photosynthesis.