. The Canadian field-naturalist. 10 The Canadian Field-Naturalist [Vol. 71 WOLFF IA IN CANADA ' William G. Dore Science Service Building, Ottawa, Ontario Received for publication, April 11, 1956. The water-meals of the genus Wolffia, the smallest of all flowering plants in the world, are of relatively restricted occurrence in Can- ada and consequently, when they are en- countered in the field, they become a matter of considerable interest. In recent years, Two of the three species of Wolffia recog- nized for North America, W. punctata Grise- bach and W. columbiana Karsten, are present in south
. The Canadian field-naturalist. 10 The Canadian Field-Naturalist [Vol. 71 WOLFF IA IN CANADA ' William G. Dore Science Service Building, Ottawa, Ontario Received for publication, April 11, 1956. The water-meals of the genus Wolffia, the smallest of all flowering plants in the world, are of relatively restricted occurrence in Can- ada and consequently, when they are en- countered in the field, they become a matter of considerable interest. In recent years, Two of the three species of Wolffia recog- nized for North America, W. punctata Grise- bach and W. columbiana Karsten, are present in southern Ontario and adjacent southwest- ern Quebec. Although about the same in size (1 to 2 mm), the two species are decidedly. â .iLi tl;^. Fig. 1. Two species of Wolffia, enlarged about 30 times, floating on water from a pond at Ottawa. The pointed plant in the upper left is W. punctata and the rounded plant in lower right of the photo- graph is W. columbiana; other individuals in various stages of growth and fragmentation are seen. A portion of a frond of Lemna minor appears in the upper right. Wolffia has been found in abundance at a few sites near Ottawa and so it has been possible to observe the plants more closely in the living state (Fig. 1). There has been difficulty, however, in identifying them to species by the use of keys and descriptions in current manuals, and this state has sug- gested the present review of their diagnostic characters and geographic distribution. 1 Contribulion No. from the Botany and Plant Pathology Division, Sricnce Service, Canada Department of Agriculture, Ottawa, Ontario. different in shape and structure of the plant body. In dry specimens, however, the char- acters are scarcely discernible and one has to rely on the presence or absence of 'dots,' or punctae. In his classical revision of the North American Lemnaceae, C. H. Thomp- son (1898) stated that W. punctata was "pro- fusely punctate throughout with brown epi- dermal cells,&qu
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