Xylem and Phloem
Xylem cells have extra reinforcement in their cell walls, and this helps to support the weight of the plant. For this reason, the transport systems are arranged differently in root and stem – in the root it has to resist forces that could pull the plant out of the ground. In the stem it has to resist compression and bending forces caused by the weight of the plant and the wind.
Stem – the xylem and phloem are arranged in bundles near the edge of the stem to resist compression and bending forces. Root - xylem and phloem in the centre of the root to withstand stretching forces.
Tissue - Process - What is moved - Structure
Xylem - Transpiration, Moves water and minerals from roots to leaves, Columns of hollow, dead reinforced cells.
Phloem - Translocation, Moves food substances from leaves to rest of plant, Columns of living cells
Water on the surface of spongy and palisade cells (inside the leaf) evaporates and then diffuses out of the leaf. This is called transpiration. More water is drawn out of the xylem cells inside the leaf to replace what's lost. As the xylem cells make a continuous tube from the leaf, down the stem to the roots, this acts like a drinking straw, producing a flow of water and dissolved minerals from roots to leaves.
Factors that speed up transpiration will also increase the rate of water uptake from the soil. When water is scarce, or the roots are damaged, it increases a plant’s chance of survival if the transpiration rate can be slowed down. Plants can do this themselves by wilting, or it can be done artificially, like removing some of the leaves from cuttings before they have chance to grow new roots.
Light - In bright light transpiration increases The stomata (openings in the leaf) open wider to allow more carbon dioxide into the leaf for photosynthesis
Temperature -Transpiration is faster in higher temperatures Evaporation and diffusion are faster at higher temperatures
Wind -Transpiration is faster in windy conditions Water vapour is removed quickly by air movement, speeding up diffusion of more water vapour out of the leaf
Humidity -Transpiration is slower in humid conditions Diffusion of water vapour out of the leaf slows down if the leaf is already surrounded by moist air