There are two types of transport vessels in plants:
The main function of the xylem is to transport water and minerals from the roots to the stem and leaves. They are made up of elongated dead cells lined end to end, forming a continuous tube. Through this tube, water is transported in a transpiration stream.
Movement in the xylem is unidirectional (one way), flowing from the roots to the leaves, with no end walls between cells.
Xylem vessels are well adapted to carry out their functions in plants. For example, they have the following features:
The phloem transport dissolved sugars and amino acids both up and down the plant through translocation.
The phloem tubes are made up of living cells. They have end walls with pores that allow dissolved substances to be transported from one cell to another. These pores are called sieve plates, which allow substances to flow through easily.
|Type of transportation||Transpiration stream||Translocation|
|Substances transported||Water and mineral ions||Sucrose and amino acids|
|Movement of substances||Upwards||Upwards and downwards|
|Nature of tissue||Dead cells without cytoplasm or organelles.||Living tissue|
|Type of process||Passive process||Active process|
The movement of water in the xylem is a passive process powered by the evaporation of water from the stomata in the leaves, which creates a negative pressure or suction that pulls water up from the roots.
In contrast, the movement of sugars, amino acids and mineral ions in the phloem requires energy. The loading of sugars into the phloem at source locations (like leaves) usually requires active transport, which involves energy.
This creates a high concentration of solutes in the phloem, causing water to move in from the xylem, creating pressure that pushes the sugars towards sink locations (like roots or fruits). This whole process, involving active transport and the pressure-driven flow of water, makes it an active process.