Edge Effect in Marram Grass

Marram grass (Ammophila spp) is a xerophytic dune- dwelling plant in Europe. It has some unique anatomical adaptations of its leaf structure which permit it to live in sandy conditions, where it stabilizes dune structure and provides a windbreak and contributes substantially to being a topographical feature in its own right. It is a great example of an microscopic edge effect which goes on to become a macroscopic edge effect.

The grass blades are curled in the manner of a drinking straw, creating a closed micro environment in which the lower epidermis is enclosed. This is significant because it is the lower epidermis which is vulnerable to water loss via transpiration. By trapping a column of moist air within the leaf blade, much less water is lost. To enhance this effect, the cuticle upper epidermis, exposed to the elements, is thickened and waxy.

But the xerophytic adaptations of Marram grass go further than that. Microscopic examination of the lower epidermis reveals a series of longitudinal involutions which replicate on a small scale the curling effect described above. This further traps water in a humid column. Additionally, the stomata (holes in which water is lost from the lower leaf surface) are sunken into pits which serve to strengthen this effect in triplicate. The whole structure is reinforced by stomatal hairs which raise water potential adjacent to the leaf surface and slow transpiration.

Taken together, the adaptation of the anatomy of the lower leaf surface in Marram grass serves to generate an edge effect by design. These small changes, amassed over each blade of grass, permit extensive colonization of the beach margin with profound impacts on coastal structure and wildlife.