DUCKWEED, the common botanical name for species of lemna (family Lemnaceae) which form a green coating on fresh water ponds and ditches. The plants are of extremely simple structure and are the smallest and least differentiated of flowering plants. They consist of a so called "frond"—a flattened green more or less oval structure which emits branches similar to itself from lateral pockets at or near the base. From the under surface a root with a well-developed sheath grows downwards into the water. The flowers, which are rarely found in Britain, are developed in one of the lateral pockets. The inflorescence is very s i m p 1 e, consisting of one or two male flowers each comprising a single stamen, and a female flower com prising a flask-shaped pistil. The family to which they belong is regarded as representing a very reduced type nearly allied to the Aroids.
Besides Lemna, other genera representative of the family are Spirodela, with several roots, and W of ffia and W of ffiella, with no roots. In Great Britain three spe cies of Lemna occur; the lesser duckweed (L. minor), the gibbous duckweed (L. gibba), and the ivy-leaved duckweed (L. trisulca) ; the greater duckweed (Spiro dela polyrrhiza) is also found. All the foregoing are distributed widely throughout the world. In Great Britain the family is further represented by Wolffia arrhiza, in which the rootless fronds are only in. long; it is the smallest of British flower ing plants. In North America eight duckweeds occur, including all those found in Great Britain; there are also three species each of W of fia and W olfella. W olfia punctata, with fronds only Ay in.
to in. long, is one of the most minute of all flowering plants.