What primarily drives the flow of water through the xylem in plants?

Atmospheric CO₂ has been increasing rapidly since the late 1800s, largely due to human activities. Recall that CO₂ enters leaves through stomata and can then be used for photosynthesis.

However, transpiration occurs as a result of water evaporating through stomata.

How have plants responded to elevated CO₂ levels? In the year 1915, the stomatal conductance of oak was approximately how many times higher than that of pine?

How about in the year 2010?

Atmospheric CO₂ has been increasing rapidly since the late 1800s, largely due to human activities. Recall that CO₂ enters leaves through stomata and can then be used for photosynthesis. However, transpiration occurs as a result of water evaporating through stomata.

How have plants responded to elevated CO₂ levels?

Assuming that the CO₂ level continues to increase with time, how likely are plants to be able to continue to adapt by adjusting stomatal conductance?