An industrial society, especially one as centralized and concentrated as that of Britain, is heavily dependent on certain essential services: for instance, electricity supply, water, rail and road transport, and harbors. The area of dependency has widened to include removing rubbish, hospital and ambulance services, and, as the economy develops, central computer and information services as well. If any of these services ceases to operate, the whole economic system is in danger. It is this economic interdependency of the economic system which makes the power of trade unions such an important issue. Single trade unions have the ability to cut off many countries' economic blood supply. This can happen more easily in Britain than in some other countries, in part because the labor force is highly organized. About 55 percent of British workers belong to unions, compared to under a quarter in the United States. For historical reasons, Britain's unions have tended to develop along trade and occupational lines, rather than on an industry-by-industry basis, which makes a wages policy, democracy in industry and the improvement of procedure for fixing wage levels difficult to achieve. There are considerable strains and tensions in the trade union movement, some of them arising from their outdated and inefficient structure. Some unions have lost many members because of their industrial changes. Others are involved in arguments about who should represent workers in new trades. Unions for skilled trades are separate from general unions, which mean that different levels of wages for certain jobs are often a source of bad feeling between unions. In traditional trades which are being pushed out of existence by advancing technologies, unions can fight for their members' disappointing jobs to the point where the jobs of other union members are threatened or destroyed. The printing of newspapers both in the United States and in Britain has frequently been halted by the efforts of printers to hold on to their traditional highly-paid jobs. Trade unions have problems of internal communication just as managers in companies do, problems which multiply in very large unions or in those which bring workers in very different industries together into a single general union. Some trade union officials have to be re-elected regularly others are elected, or even appointed, for life. Trade union officials have to work with a system of 'shop stewards' in many unions, 'shop stewards' being workers elected by other workers as their representatives at factory or works level. Why is the interdependence of the U.K. economy mentioned in paragraph 1?