BOT 2100 Lecture Notes - Lecture 19: Vessel Element, Phloem, Physical Chemistry
Document Summary
Describes how water moves through the xylem passively due to the cohesive properties of water. As plants transpire, more tension is created as water moves upwards. Water uptake is higher during the day due to higher rates of transpiration. In theory, a water column over 300m can be maintained by cohesive forces. Flow rate in xylem is normally 1-5m/h but can reach up to 45m/h in some plants. Water is pulled from root to xylem, then continuous into leaf tracheary elements. In tall trees, the center shrinks during the day as tension builds, and expands at night. Tracheary elements are reinforced by lignified secondary walls that allow for pressure to build without imploding. Cavitation: the introduction of air into the xylem (embolism). This can cause issues in maintaining tension and cohesion. In this case, water often moves around the bubble into adjacent columns.
