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The first edition of this handbook contained a review of the literature concerned with the utilization of fresh-water aquatic plants going as far back as 1917. The objective was to assemble the ideas and experimental results of those authors and research workers who considered that aquatic plants could, or should, be looked upon in one way or another as useful crops. That this concept was rather novel at the time was indicated by the relatively small number of publications found on the subject. A total of 49 references was listed in the table of contents of which only about 30 were complete papers; the remainder were articles taken from papers or books on wider or related topics. Inevitably a few papers were overlooked.

Since 1968 about five times as much information on the use of aquatic weeds has been released as during the whole previous history of the literature, making a revision of the first edition well worth while. The large number of references also makes it impracticable to reproduce papers more or less verbatim, as in the first edition.

An attempt has been made here to summarize the published material as well as to review it. Tabulated material has, in many cases, been condensed or rearranged and analyses have been selected from the considerable amount of data available. In those cases where the complete paper has not been available, frequent use has been made of the invaluable Commonwealth Agricultural Bureaux publication, Weed Abstracts. There is a danger of presenting extracts which, read out of context of the complete paper, may perhaps be interpreted rather differently. This risk has been accepted, and sufficient information is given to reveal the range of each paper.

More than 250 papers are cited, including those in the first edition (which are marked *). The annotated references are given in alphabetical order of authors within each chapter.

The handbook has been divided into chapters on plant analyses, methods of harvesting, and various aspects of use of aquatic plants. No attempt has been made to cover the age-old uses of aquatic plants such as for weaving and thatching, and only brief reference is made to industrial uses such as for paper making, as these aspects are well covered in the established literature. For example, Boyd (1974) has summarized the uses of reeds which represent, so far, the most heavily utilized aquatic plant. In Romania, where the industry is most fully developed, harvests have reached 200 000 t/year (Rudescu, Niculescu and Chivu, 1965).

This edition also includes a section on water purification. Many water weeds respond to the high quantities of nutrients such as nitrogen or phosphorus in polluted water through greatly increased growth. This is generally viewed with dismay as the vegetation can create various nuisances, but now, with pollution becoming an increasingly serious problem itself, there is an awareness that aquatic plants may be useful as a means of extracting contaminants from water. They could provide an alternative to more expensive, conventional means of water purification and at the same time be exploited as a resource, thus paying for the costs involved. Toxic industrial pollutants, e.g., heavy metals, may also be extracted from water by plants. So the proposals of early enthusiasts that aquatic plants, used in this way, could be classed as useful crops rather than notorious weeds are being increasingly accepted.

A survey of the losses caused by aquatic weeds has been made by the United States Agency for International Development (1971) in Ghana, Sudan, Zambia, Thailand, Guyana and the U.S.A. The survey points out how serious the need to control aquatic weeds may be for developing countries which are not able easily to absorb the costs of control. Thus to be able to convert such concentrations of plants to useful purposes instead of just accepting the costs of their destruction could be of considerable importance to the economies of such countries.

A method of assessing losses due to aquatic weeds, based on measurement of water reduction and the consequent value of loss of crop, has been described by Timmons (1971). This is one way to help in arriving at a decision as to how much expenditure is worth while in attempts to control the weeds and whether the costs of harvesting for utilization are justified.

The water hyacinth, Eichhornia crassipes, is the best known and most notorious aquatic plant. It has received by far the most attention in the literature because this extraordinary tropical weed is more widespread than any other.

In the Sudan the water hyacinth is of exceptional importance, threatening as it does the navigability of a main artery of the country, the White Nile. In 1975 the Sudan National Council for Research, Agricultural Research Council, in cooperation with the United States National Academy of Sciences, organized a workshop on the management of water hyacinth, of which its utilization was an important aspect. Delegates from Egypt, Ethiopia, Indonesia and Mozambique participated.

A similar workshop in Guyana was organized in 1973 by the National Science Research Council in cooperation with the United States National Academy of Sciences. Methods of processing water hyacinth for animal fodder and for fertilizer were discussed. The utilization of Pistia stratiotes was also discussed.

A detailed study of how aquatic plants could be usefully employed to play a part in the agricultural schemes of rich and poor countries has been advocated by a few writers for many years, and this interest has increased. Many plants, once seeming to threaten agriculture by impeding water flow and wasting water, may be shown to have characteristics which will enable them to provide a valuable complement to agriculture.

It is hoped that this handbook will supplement the literature by helping those who wish to control and to make use of these plants, especially the most dangerous ones, and thus convert them from enemies to useful friends.