One fundamental difference between plants and animals is that animals can move from one place to another, whereas plants lack mobility. But this is not true for all kinds of plants, because there are some plants which move on their own. For example, slime moulds have amoeba-like movement by which they move from one place to another. Some types of algae have whip-like flagella, which they use to paddle themselves through water. Many plants, particularly, the lower one’s, produce mobile male gametes that swim about in order to find eggs to fertilise. Euglena is a protozoan, which is capable of swimming. Apart from these exceptions, the movement of plants is usually confined to the movement of some parts of it while the plant itself remains fixed at one place. There are three basic types of plant movements — tropisms, nutations and nastic movements. Tropism is a growth response towards or away from something caused by a specific environmental stimulus. The direction of growth is determined by the stimulus. When it is towards the stimulus, it is called positive tropism and when away from the stimulus, it is known as negative tropism. Tropisms are caused by special growth hormones called auxins. In most cases, the stimulus causes the auxins to accumulate on one side of an affected organ. This causes the cells on that side to grow and divide more quickly than the cells on the other side. As a result, the organ moves away from the side with the maximum auxins. There are several types of tropism. Phototropism is a growth response to the stimulus of light. Auxins are concentrated on the side away from light. This causes stems and leaves to grow towards light and roots to grow away from light. Geotropism is the growth due to the Earth’s gravity. Roots show positive geotropism, while stems show negative geotropism. Similarly, hydrotropism is the growth response of a plant to the stimulus of water. Roots grow towards water and often cover great distances to reach areas of moist soil. Nutation is the spiral movement exhibited by the tips of certain stems during growth, which enables a climbing plant to find a suitable support. Nastic movement is caused by an external stimulus, such as light or temperature, but the movement is independent of the source. This movement is neither towards nor away from a stimulus. For example, some flowers open in daylight and close at night. You must have noticed the leaves of touch-me-not (mimosa pudica) droping when touched by hand or stick or when an insect sits on it. This is also an example of a nastic movement.