Nerve cells pass electrical impulses along their length. They stimulate their target cells by secreting chemical neurotransmitters directly on to them. This results in rapid, short-lived and localised responses. Mammalian hormones are substances that stimulate their target cells via the blood system. This results in slow, long-lasting and widespread responses. Histamine and prostaglandins are local chemical mediators released by some mammalian cells and affect only cells in their immediate vicinity. In flowering plants, specific growth factors diffuse from growing regions to other tissues. They regulate growth in response to directional stimuli. The role of indoleacetic acid (IAA) in controlling tropisms in flowering plants.

There are some chemical mediators which are released at the site which needs a response. In animals these are histamine (involved in immune and allergic response) and prostaglandins (involved in response to injury and illness).

In plants chemicals used to control growth, in terms of direction, are released from the growing root or shoot and diffuse down the chemical gradient to other parts of the plant. IAA is an example of this: it is released from the growing shoot and causes cells to elongate, however it is no present in direct sunlight, which means that the side of a plant with the most sun will not grow in comparison to the rest of the plant, curving it towards the light.