Transpiration stream

In the syllabus and the exam

Transpiration stream refers to the continuous, unbroken column of water that moves from the roots to the leaves through the xylem, and this term is used to describe the whole transport pathway as a single connected system rather than any one stage in isolation. Candidates are expected to trace this pathway in the correct order in structured questions: absorption by root hair cells, movement across the root tissues into the xylem, upward transport through the xylem in the stem, and finally movement into the leaf mesophyll before evaporation and diffusion out through the stomata. The driving force behind this upward movement is transpiration itself, which creates a pull, sometimes described using terms such as suction or tension, transmitted down through the continuous water column inside the xylem vessels. A frequently tested idea is why the xylem's structure, being dead, hollow, and lignified, is well suited to withstanding this tension without collapsing. A common error is describing the transpiration stream as being pushed up from the roots rather than pulled from above by water loss at the leaves, or omitting one of the intermediate stages, such as the root cortex, when tracing the full pathway in an examination answer. Candidates should also be able to explain what would happen to the transpiration stream if an air bubble entered the xylem, breaking the continuous water column, since this is occasionally used as the basis for a structured question testing whether candidates understand that the pull relies on an unbroken chain of water molecules. This is also why cutting a shoot underwater before placing it in a potometer is standard practice, since it prevents an air bubble from being drawn into the cut xylem vessels, which would otherwise break the water column and stop the stream entirely.

Example

Describing the pathway of water, a candidate writes: 'The transpiration stream carries water from the root hair cells, across the root cortex, into the xylem, and up through the stem to the leaf mesophyll cells, where it evaporates and diffuses out.'

Related terms

TranspirationXylemRoot hair cell

Source:SRC-DSKP-EN