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EARLY land plants evolved broad leaves to cope with plummeting levels of
carbon dioxide in the atmosphere, biophysicists believe.

The first land plants were spindly, branching twigs when they appeared about
400 million years ago. Eventually some evolved into giant asparagus-like trees
up to 30 metres tall, but leaves remained thin, spine-like structures. Only
about 360 million years ago did broad leaf surfaces begin to fill in the gaps
between the branches.

David Beerling of the University of Sheffield and his colleagues realised
that this transformation coincided with a 90 per cent drop in atmospheric carbon
dioxide. Beerling鈥檚 team compared fossils of the earliest leafless plants with
their broadleaved descendants and found that plants evolved broad leaves as they
cranked up the densities of their stomata, the pores through which plants absorb
CO₂for photosynthesis. Early twiggy plants needed few stomata because
of the high levels of CO₂. But as CO₂ levels dropped, plants
needed one hundred times the density of pores to suck in enough gas to
survive.

Plants also regulate their temperature through water vapour evaporating from
their stomata. Only after plants had begun evolving more stomata could broad
leaves lose enough water to keep sufficiently cool in tropical areas, Beerling
says.