1.   Ethylene is a small hydrocarbon gas.
2.   It is a plant hormone which is synthesized during the ripening of fruits.
3.   Ethylene is responsible for the changes in texture, softening, color, and other processes involved in ripening. 
4.   Ethylene
(a) Speeds up the ripening of fruits by stimulating the production of cellulose. Cellulase hydrolyses the cellulose in plant cell walls, making the fruit soft.
(b) Promotes the breakdown of complex carbohydrate into simple sugar. This make ripe fruit tastes sweeter than an unripe fruit.
5. Ethylene gas is used commercially to ripen tomatoes, bananas and pears.

3.6 Plant Hormones

3.6 Plant Hormones
1.      Growth of shoots towards sunlight is called positive phototropism.
2.      Growth of shoots away from sunlight is called negative phototropism.
3.      In plants, hormones play an important role in growth processes such as seed germination, growth of roots and development of fruits.
4.      There are many types of plant hormones, such as auxins and ethylene
Effects of Auxins on Growth Responses
1.      Auxins are produced in the apical meristems of the shoot tip and the root tip in the cell division zone.
2.      In the shoot tip, high concentrations of auxins stimulate cell elongation.
3.      The effect of high concentrations of auxins in the root tip is different from that in the shoot tip. In the root tip, a high concentration of auxins inhibits cell elongation.