The ribbed underside of the Amazon water lily provides structural support to keep the leaf afloat and sustain small loads.

Many plants that grow near the banks of rivers will float their leaves on the surface, to allow their seeds to be carried downstream and pollinate other areas. In order to ensure a long trip, leaves that are able to float on the surface can ensure maximum sun absorption for . The larger a leaf is, the more surface area it can expose for photosynthesis, and the more aggressively it can compete with other leaves for space on the surface of the water. One of the most extreme examples of this is the leaves of the Amazon water lily, which float on the water of the Amazon river basin and grow up to several meters across. Its massive surface area allows it to maximize sun exposure, but also makes it more difficult to keep afloat and makes it susceptible to damage from animals that use the leaves as a means to get across the river. To cope with this, the Amazon water lily has a significant support system on the underside of its leaves, which allow it to sustain weights up to 70 pounds, while still remaining afloat.

Underneath each leaf is a ribbed, girder-like support structure that helps the leaf support small loads and hold a rigid shape, in order to maximize surface area exposure for photosynthesis. The structure consists of a main rib that runs along the center of the leaf, with additional ribs that radiate from the center and incrementally fork along the leaf. The ribs themselves are flat, wall-shaped structures, and the thickness of the ribs decreases towards the edge of the leaf. All of the ribs are filled with air, which helps to reduce the total weight of the structure and also helps keep the leaf afloat. Neighboring ribs are connected to each other by a pattern of radial webbing forming loose concentric circles that emanate from the center of the leaf. This webbing provides additional structural support without adding excess weight.

Image: Peter from Perth / CC BY NC ND ‑ Creative Commons Attribution + Noncommercial + NoDerivatives

Girder‑like support ribs of Amazon water lily

Image: Canon EOS 650d / Public Domain ‑ No restrictions

Amazon Water Lily 

Image: Alex Ralevski / Copyright © ‑ All rights reserved

Drawing of the underside of the Amazon water lily reveals intricate support structure

Image: Laitr Keiows / CC BY SA ‑ Creative Commons Attribution + ShareAlike

Ribbed underside of Amazon water lily shows support structure 

Last Updated March 25, 2020