Why plants need a transport system
A tall tree may stand many metres high, yet its roots absorb water deep in the soil while its leaves make food high in the air. Diffusion alone is far too slow to move substances across such distances, so flowering plants use a network of specialised tubes called vascular tissue. This tissue is grouped into vascular bundles that run through the roots, stem and leaves.
Key idea
Xylem carries water and dissolved minerals upward from the roots to the leaves. Phloem carries dissolved food (mainly sucrose) made in the leaves to all parts of the plant.
Absorption and the transpiration pull
Water enters through thin-walled root hair cells, whose long shape gives a large surface area for absorption by osmosis. The water then passes into the xylem. In the leaves, water evaporates from cell surfaces and escapes as vapour through tiny pores called stomata. This loss of water is transpiration, and it creates a suction called the transpiration pull that draws a continuous column of water up the xylem.
Example
On a hot, dry, windy day transpiration is faster, so water is pulled up the xylem more quickly and the plant absorbs more water from the soil.
Moving food: translocation
Sugar made during photosynthesis is transported through the phloem to growing buds, roots and storage organs. This movement is called translocation, and unlike the one-way flow in xylem it can go both up and down the plant.
Remember
- Xylem = water + minerals, upward only, dead hollow cells.
- Phloem = dissolved food, both directions, living cells.
- Transpiration pull lifts water; root hair cells absorb it by osmosis.