Representing hundreds of thousands of species and 96% of all terrestrial vegetation, flowering plants are the most successful land plants on Earth. How did orchids, daisies, and their ilk become so dominant? Researchers have long chalked it up to their flowers, which enlist insects and other animals to help them reproduce and spread. But two plant biologists credit the leaves instead. More leaf veins made the plants better photosynthesizers, the duo reports, enabling angiosperms to outgrow their competition.
The two researchers--Timothy Brodribb, a hydraulic physiologist at the University of Tasmania in Australia, and Taylor Feild, now at the University of Tennessee, Knoxville--were studying how leaves transport water. They noticed that the leaves of early angiosperms seemed to contain far fewer veins than those of angiosperms that evolved later. "We were shocked by the extraordinary contrast," Brodribb recalls.
Their work has shown that the density of veins is important to the ability of the plant to photosynthesize. To grow, plants must take in carbon dioxide through valves in the leaves called stomata. When open, stomata lose water, so the more efficient the transport of water to replenish lost moisture, the more stomata that can stay open and take up carbon dioxide. The researchers wondered if the evolution of more veins per leaf gave angiosperms the boost they needed to become widespread.
Based on fieldwork in 13 countries over 6 years, the duo examined the venation of 504 flowering plants and 225 other plants, including 166 extinct species, and looked for trends in venation patterns through time. In addition, they measured water and carbon dioxide exchange in leaves from 35 species of flowering and nonflowering plants.
"The evidence for a transformation in venation was remarkably clear," says Brodribb. Early angiosperms had simple leaf patterns with few veins. But about 100 million years ago, newer species of angiosperms had doubled, tripled, and, ultimately, increased by 10-fold the number of leaf veins, the team reported  online last week in Ecology Letters.
According to the researchers' model of plant physiology, a tripling of vein density would have doubled the plant's photosynthetic capacity. Brodribb and Feild argue that more photosynthesis meant more carbon for growth. And that would have given the angiosperms the energy to push competitors like conifers out of the canopy around 150 million years ago, making angiosperms the most productive group of land plants in the world. Flowers play a role in the diversification, the team says, but leaf veins help these plants become so abundant.
"The importance of vein density has never before been so clearly presented," says Peter Wilf, a paleobotanist at Pennsylvania State University, University Park. "The work provides the first quantitative, physiological, and phylogenetic framework for understanding why, when, and how angiosperms evolved much higher photosynthetic rates than other plants." This paper will likely be very influential, adds Lawren Sack, a plant physiologist and ecologist at the University of California, Los Angeles, as paleobiologists can now better estimate photosynthesis for fossils from deep time.