The complex cross-pollination mechanisms were described by Charles Darwin in his 1862 book "The Fertilisation of Orchids". Orchids have developed special pollination systems. The chances of being pollinated are often scarce, so orchid flowers usually remain receptive for very long periods, and most orchids deliver pollen in a single mass. Each time pollination succeeds, thousands of ovules can be fertilized. "Catasetum", a genus discussed briefly by Darwin, actually launches its sticky pollinia with explosive force when an insect touches a seta (hair), knocking the pollinator off the flower.