Herbaria are vital sources of comparative data for botanists and paleobotanists alike, especially those of us interested in patterns of leaf venation. Traditionally, one way that paleobotanists have obtained images of leaf venation from herbarium sheets was to remove a leaf from the sheet, clear it until the tissue is translucent, and then stain the cleared leaf with safranin. Safranin is a dye that binds to the lignin in plant tissue, providing contrast between the veins and the leaf mesophyll. Then, the stained leaves are mounted on glass slides. There are already large collections of cleared leaves around the world, including at the Smithsonian, Yale, Berkeley, and the Florida Museum of Natural History.
Leaf clearing can give great results, but the process is time-consuming and delicate, as the brittle, dried leaves become flimsy and prone to tearing when they are cleared. Once the slides are made, they may also need to be maintained, depending on the mounting medium.
When I was working on Potomacapnos, I found that some of the leaves that I wanted to compare were not represented in the cleared leaf collection. I cleared some leaves myself, but I also experimented with alternative ways of imaging leaf venation. To my surprise, the most successful method turned out to also be the easiest. I started photographing leaves directly on herbarium sheets, but with a DSLR camera and powerful flash from below. The light passes right through most moderately thin or very thin herbarium sheets and through the leaf-of-interest with sometimes spectacular results (Jud and Hickey, 2013). I recently visited the Smithsonian again to take some new photos of leaf venation for a forthcoming project. Here is one example of the results:
