132 FLOWERS OF FIELD, HILL, AND SWAMP 



This family comprises our smallest flowering plants. The 

 leaves are less than half an inch across. The tiny plants cover 

 the water's surface at the time of their vegetating, in spring and 

 summer. Dr. Gray says of the duckweeds that they are propa- 

 gated " by the proliferous growth of a new individual from a cleft 

 in the edge or base. of the parent frond, remaining connected for 

 some time, or separating ; also from autumnal fronds in the form 

 of minute bulblets, which sink to the bottom of the water, but 

 rise and vegetate in spring ; the flowers and fruit scarce, in some 

 species never seen." 



There are 3 genera and 26 species, found in rivers, ponds, pools, 

 and shallow bays from Maine to North Carolina. Limna trisiilca 

 has lance -shaped thalli, narrowed at the base. L. perpusUla, 

 with thalli scarcely bigger than a pin's head, is found in New 

 York and New Jersey ponds. Wblffia is the third genus. Two 

 species are found with us, floating like minute grains just under 

 the water's surface. They come almost within the range of mi- 

 croscopic plants. 



