Fruit and seed structural characteristics and seed dispersal in Mercurialis annua L. (Euphorbiaceae)

The fruit of Mercurialis annua L. is a two-seeded capsule with a caruncle, a small appendage which arises from the outer integument in the micropylar area. The inner integument has a thick layer of Malpighian-like cells, interrupted at the micropyle by a remnant of nucellus, the walls of which contain suberin. There is no cuticle covering the caruncle and its cells have lipids and protein bodies as reserves. Seed dispersal occurs by a combination of autochory followed by myrmecochory. The explosive ejection of seeds is caused by dehydration-induced torsion of the mechanical layer of the fruit walls. This phenomenon is due to the different orientation of the cells and a folding over of the mechanical layer in the chalazal area. The seeds were found to be thrown distances from 1 to 130 cm (mean 41.1 cm; modal peak 10-20 cm). Ballistic dispersal is influenced by the weight of the seed. The seeds were collected by ants in a mean time of 24.4 minutes the maximum and mean distances of dispersal being 14 and 3.4 m, respectively. Of the seeds removed, 95.2 % were recovered on excavation of the ant nest. The seeds in the nest were intact but without the caruncles. The mean distance of dispersal by diplochory was < 5 m. These features are discussed in relation to possible benefits to the plant species in terms of adaptive advantages of seeds