Bacteria also come within the division of thallophytes that are dependent on green plants, for they are a group of minute organisms without any cholorophyll. They are parasites or saprophytes and are usually single celled often with the power of movement in the same way as chlamydomonas. They increase by straightforward division of an old cell into two new ones, or they can also form resting spores. These consist of cells which have specially tough cell walls, and in this stage the bacteria is very resistant, in some cases even to boiling water. These spores are very numerous and are present in the soil, the air and water. In fact they are present virtually everywhere unless special precautions are taken to kill them. During the resting stage they are not increasing at all but when conditions are suitable for their growth the cell wall breaks down and the bacteria become active again. Parasitic bacteria are often harmful such as anthrax in animals tetanus in man and soft roots in plants but quite a number do exist on other living organisms without doing them any harm. Saprophytic bacteria are often useful for they help in rooting down dead material. They are very active in for example a compost heap. Some are used to covert alcohol into vinegar. One group of bacteria (nitrogen fixing bacteria) are especially important as they live in swellings in the roots of plants of the legume family (peas and beans) and have the power of extracting nitrogen from the surrounding air in the soil and making it into a form that can be used by plants. This is why plants of this family, especially clover, are often grown in fields and then ploughed in to improve the soil. Other bacteria in the soil (nitrifying bacteria) can covert plant and animal remains into a form that plants can use. So by the very valuable action of these tiny life forms a continuous process is set up of turning waste and dead materials back into food for other plants. This called the nitrogen cycle.