Plant Leaf (IMAGE) University of Washington Caption This closeup of a leaf was taken on Barro Colorado Island in 2013. Plant leaves tend to thicken under higher CO2, but other factors, like the concentration of nitrogen in the leaves, also affect the plant's response. Credit Brian Gratwicke/Flickr Usage Restrictions With credit License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.