Ethylene is one of the only gaseous hormones in plants, however it modulates vital plant functions. Ethylene is produced in all higher level plants from methionine in essentially all tissues of the plant. This hormone is a major stimulant throughout the various stages of plant growth. During spring and summer ethylene is primarily used by the reproductive systems of plants – in flower opening, and fruit ripening. In autumn, the hormone is believed to trigger the abscission (shedding) of leaves. Ethylene is transported throughout the plant by the use of air, which is why fruits in a container can ripe faster (because more ethylene is concentrated in the sealed container). The ripe apple in the picture above ripened due to ethylene.