One of the fundamental differences between plants and animals is that animals can move from place to place whereas plants lack mobility. But inconsistent with this general distinction, there are some plants which move on their own. For example, slime molds have amoeba like movement whereby they ‘coze’ from one place to another. Some types of algae have whip like flagellas, they use to paddle themselves through water. Many plants particularly, the lower ones, produce mobile male gametes which swim about in order to find eggs to fertilize. Englena is a protozoa 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.
A 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 caller positive tropism and when away from it, it is called negative tropism.
Tropisms are caused by special growth hormones called auxins. In most of the cases, the stimulus causes the auxins to collect on one side of an affected organ. This causes the cells on that side to grow and divides more quickly than the cells on the other side. As a result, the organ bends away from the side with the most auxins.
Tropism is of several types. Phototropism is a growth response to the stimulus of light and auxins are concentrated on the side away from the light. This causes stems and leaves to grow towards light and roots grow away from light. Geotropism is the growth towards the gravity of earth. Roofs show positive geotropism while stems show negative geotropism. Hydrotropism is the growth response to the stimulus of water. Roots grow towards water and often move great distances to 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. For example, some flowers open in daylight and close again at night. Flowers, like daisy open only when the temperature is just right.