A colourless gaseous hydrocarbon, C₂H₄, that occurs naturally in plants and acts as a plant hormone in a variety of physiological roles. It is produced in response to stresses, such as drought, flooding, mechanical injury, or infection, and has a key role in senescence, fruit ripening, and leaf abscission. Auxins stimulate tissues to produce ethylene, which diffuses rapidly to trigger responses in surrounding cells. The best known effect is the stimulation of fruit ripening: fruits such as bananas, apples, and avocados naturally produce ethylene during the later stages of ripening, and ethylene gas is used to promote the ripening of fruits, such as bananas, that are picked and shipped ‘green’. The orderly programmed cell death (apoptosis) that occurs during senescence is prompted by a burst of ethylene production, and ethylene triggers cells to produce the enzymes responsible for digesting the abscission layer to allow leaf fall. Ethylene generally suppresses flowering, except in members of the pineapple family (Bromeliaceae)—hence flowering of pineapples may be synchronized by releasing ethylene into the growing crop. Studies have shown varied and often contradictory effects of ethylene on vegetative growth. For example, in rice it acts with gibberellins to promote stem elongation, while in peas ethylene inhibits root and shoot elongation. Seed germination, bud opening, and root initiation may also be promoted by ethylene.