Final answer:
When less water is available, smaller xylem cells with thick walls known as tracheids form, creating dense late wood as part of the tree's annual rings, adapting the tree to withstand drier conditions and minimize cavitation risks.
Step-by-step explanation:
When there is less water and smaller xylem cells with thick walls form, this typically results in the creation of late wood, or autumn wood, as part of the tree's annual rings. Annual rings are formed due to the seasonal activity of the vascular cambium, which produces cells of the secondary xylem that are denser and have thicker cell walls in the fall season compared to the spring. These thicker-walled cells are known as tracheids, which are a type of xylem cell with thick, lignified secondary cell walls that help the tree withstand the increased tension from the pull on water during drier conditions.
The structural adaptations of xylem, like rings that maintain the tubular shape akin to a vacuum cleaner hose, support the plant's ability to transport water efficiently in various environmental conditions. In conditions of drought or less water availability, the plant adjusts by reducing the diameter of its xylem cells and increasing the thickness of their walls. This reduces the risk of cavitation and embolisms, which occur when air bubbles form and interrupt the continuous water column within the xylem, potentially damaging the tree's ability to transport water from roots to leaves.