Tractor mowing green field, aerial view (IMAGE) Cambridge University Press Caption Tractor mowing green field, aerial view Credit Valentin Valkov Usage Restrictions Release information: Signed model release on file with Shutterstock, Inc. 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.