The arrangement of vascular bundles in the leaf (i.e., vascular topology) is widely diverse across plants, and it is known to affect the transport capacity of water across the leaf and into evapotranspiration sites.

The transport of water and sugars in leaves are functionally coupled, as some fraction of the water delivered into leaves must drive the export of sugars out of them. So, how do leaf venation patterns affect sugar transport capacity?

I compared leaves with two types of very different venation patterns: a hierarchical reticulate venation pattern (Poplar) and open dichotomous venation (Ginkgo). Phloem is the tissue that moves sugars around the plant, and because it is composed of living cells (vs. xylem, in which conducing cells are dead) it is difficult to monitor and quantify sugar export. One way around this is taking a biophysical approach and creating a model based on the conduit dimensions of the phloem along the leaf.

After hundreds of cross-sections visualized and quantified using light, confocal and transmission electron microscopy as a Ph.D. student at Cornell, a clear functional difference between these two leaf types became evident. Whereas the total conductive area of the phloem increased towards the petiole (i.e., along the sugar export pathway) in Ginkgo, the opposite occurred in poplar. The total ‘catchment area’ for sugars (comprised by the smallest veins) was much larger than the transport area at the level of the petiole…. Effectively, the transport pathway for sugars in Poplar looks like a funnel.

We hypothesized, based on first principals, that a hierarchical-reticulate topology may confer a benefit for sugar export compared to other types of venation patterns.

From an evolutionary perspective, this idea results interesting. Leaves with some form of hierarchical-reticulate venation have evolved various times, although the most conspicuous example is that of flowering plants. Does this type of venation confer a benefit for sugar export in other taxa, such as the polypod ferns? Could this approach help us understand more about the paleobiology of extinct plants we only know as fossils?