The biologists found a distinct difference in function between the major veins, which tend to show a branching pattern, and the minor veins, which form a grid embedded within the leaf and make up most of the leaf's total vein length. Blocking the major veins had a huge impact on leaf function—but one that could be remedied by having additional, redundant major veins.
Scoffoni likens the major veins to a superhighway and the minor veins to sinuous city roads, where embolism is like an accident causing a major slowdown.
"If an air bubble forms in the leaf's water pathway, the more alternate highways the vein system has to offer, the less the leaf will be affected by these accidents," Scoffoni said.
The UCLA biologists—including co-authors Michael Rawls, an undergraduate student, and Athena McKown, a postdoctoral scholar in ecology and evolutionary biology—tested diverse leaves from very wet and dry areas, all planted near the UCLA campus. The leaves fit the pattern: The biologists found that smaller leaves indeed had more tightly packed major veins and were more resistant to the effects of embolism in the major veins. The were better able to maintain water transport, even during extreme drying, Sack said.
While the trend of smaller leaves in drier areas is so striking that it appears in textbooks, and the trend is used by scientists to estimate rainfall in the distant past from the size of fossil leaves, the mechanism had never been explained. The previous theory proposed an indirect linkage, arguing that smaller leaves have a thinner layer of still air around them, which allows them to cool off faster in hotter places. According to this theory, because many dry places are also warmer, this might lead to the evolution of smaller leaves in such environments.
As Sack noted, however, "this is indirect and does not explain the trend of smaller leaves in drier places when temperature is similar. This trend appears across species, and even within individual species, when plants are grown in moister and drier soil."
The team expects that this mechanism, which points to a new role of vein architecture and leaf size in drought tolerance, will generate new interest in plant diversity and adaptation to environments. In addition, Sack said, the discovery shows that even very well-known biogeographic trends are open to new scientific explanation.