Plant hormones are signaling molecules that are produced in small amounts in one part of the plant and are transported to other parts. They can trigger specific responses involved in plant growth and development. Two major plant hormones, also called phytohormones, include the auxin and cytokinin family of chemicals. Auxins promote stem elongation and root initiation. Auxins were famously shown by Darwin to be involved in phototropism—the movement of plants towards or away from light. For example, sunflowers respond to light by tracking the movement of the sun. In response to light, auxins accumulate on the shaded side of a plant. This leads to cell elongation and bending of the plant towards the light. Cytokinins are another group of phytohormones that promote plant cell division. Cytokinins act along with auxins to mediate plant morphogenesis, which is the formation of different structures of the plant such as leaves, shoots, and roots. Another important phytohormone is ethylene, which is a gaseous plant hormone. Ethylene mediates abscission in plants—the detachment of leaves or fruits—as well as the ripening of fruits. The effects of ethylene include the enzymatic breakdown of cell wall components resulting in softening of the fruit, an increase in sugar content, and a change in the color of the fruit. Many plant hormones are very useful for agricultural practices. For instance, auxin is used in the generation of roots in the vegetative propagation of plants. Fruits are often transported in an unripened state and exposed to ethylene to quickly induce ripening once they reach their destination far from where they were harvested.