In two recent articles, we learn that not only did they help stop the earth from turning into a giant ball of ice (by regulating Carbon Dioxide levels in the atmosphere) but they also help us maintain a calm, balanced lifestyle in our lives today.
See this New Scientist article with studies about plant life effecting the atmosphere of our planet through the ages. Also see this Alternet article about ecopsychology, which recommends that we reconnect with plants, nature and by association, our own bodies.
On behalf of our species, I'd like to give a big shout out to plants. Thanks!