evolution: on the origin of leaves

The amount of CO2 in the atmosphere may have played a significant role in the evolution of leaves, report British scientists. Plants need CO2 to live. They absorb the gas through pores called stomata, which also work to cool the plant by releasing water vapor. When plants first evolved hundreds of millions of years ago, atmospheric levels of CO2 were very high, so they didn’t require a lot of stomata to survive. At this time, scientists think, large-leaved land plants couldn’t have existed: with their leaves absorbing large amounts of solar energy and with few stomata to cool them off, they’d be prone to lethal overheating. But as CO2 levels declined more than 300 million years ago, plants needed to increase the number of stomata to compensate. With more stomata, larger leaves could develop because they could be more adequately cooled. In this week’s Proceedings of the National Academy of Sciences Online Early Edition, C.P. Osborne of the University of Sheffield and colleagues decided to test this theory by examining 300 fossilized plants of various species from that period. They found that the first abrupt increase in leaf size was accompanied by an 8-fold rise in stomatal density. They also discovered that the size of the average leaf increased 25-fold between 340 and 380 million years ago, at the time CO2 levels were dropping in the atmosphere, suggesting that high levels of CO2 were a barrier to leaf development.

This news brief appeared in the Discoveries column of the Boston Globe’s Health/Science section on 7/06/2004.