  Leaves are the organs that intercept light and create photosynthesis.
Efficient light interception is provided by leaves oriented orthogonal to most
of the sun rays. Except in the polar regions, this means orthogonal to the
direction of acceleration due to gravity, or simply horizontal. The leaves of
almost all terrestrial plants grow in a gravity field that tends to bend them
downward and therefore may counteract light interception. Plants thus allocate
biomass for self-support in order to maintain their leaves horizontal. To
compete with other species (inter-species competition), as well as other
individuals within the same species (intra-species competition), self-support
must be achieved with the least biomass produced. This study examines to what
extent leaves are designed to self-support. We show here that a basic
mechanical model provides the optimal dimensions of a leaf for light
interception and self-support. These results are compared to measurements made
on leaves of various giant monocot species,especially palm trees and banana
trees. The comparison between experiments and model predictions shows that the
longer palms are optimally designed for self-support whereas shorter leaves are
shaped predominantly by other parameters of selection.
