Adaptations to Foliar Absorption of Faeces: a Pathway in Plant Carnivory

Co-habitee leaves deposit for cuticular transfer

Insectivorous plants come in many forms but the essentials of the habit are regarded as the ability to capture insects and the possession of enzymes to digest the prey. However, Bruce Anderson, , (pp. 757�761) describes a different mode of insectivory. The leaves of plants in the genus Roridula possess sticky traps that capture large numbers of insects. However, these insects are not digested by the plant because it does not possess digestive enzymes. Nevertheless, plants have been shown to take up insect nitrogen. How does this happen? Previous work had suggested that a carnivorous hemipteran insect, Pameridea, that lives obligately in close association with the Roridula plants is essential for nitrogen transfer. If Pameridea is excluded, insect nitrogen is not taken up by the plants. Pameridea devour the prey caught by the plant and the hypothesis is that the plant receives nitrogen from the trapped prey via the carnivorous hemipteran. The route of transfer is not immediately apparent until one realizes that the hemipteran deposits its liquid faeces on the leaf surface. However, there is still a problem: the cuticle appears to place an impenetrable barrier in the way of nutrient uptake from the faecal material. The author has therefore investigated the ability of Roridula leaves to take up neutral red dye. Mature leaves are uniformly permeable to the dye, in contrast to young leaves and to leaves of non-insectivorous species from the same region. This ability to absorb the dye is attributed to the thinness and irregularity of the cuticle and to discontinuities in the cuticle. Younger leaves of Roridula and all leaves of non-insectivorous plants have thicker, intact cuticles. Further, Roridula epidermal cell walls are invaginated, thus providing a greater surface area for uptake. The author concludes that Roridula species are truly insectivorous, achieving this status via a remarkable obligate mutualism.