Wolffia

Measurements of fresh material will run somewhat larger than those given for dry, but these plants are much more easily identified when fresh. Members of this genus are the smallest known flowering plants—though they are seldom seen in flower.

1. Plants longer than broad (± oblong or shaped somewhat like segments of an orange), seldom over 0.2–0.5 mm broad by 1 mm long, flattish on upper side when fresh.

2. Plants pointed at the ends (looking from above); upper surface lacking papilla.

W. borealis

2. Plants rounded at the ends (looking from above); upper surface with a prominent pointed papilla in the center of the mature plant body.

W. brasiliensis

1. Plants ± rounded in outline, the largest in a population 0.5–0.8 (–1.2) mm in diameter (much shriveled and often less than 0.5 mm if not dried under pressure, but still spherical, not oblong, in general outline); spherical when fresh.

W. columbiana

All species found in Wolffia

Wolffia borealisDOTTED WATER MEAL 
Wolffia brasiliensisPOINTED WATER MEAL 
Wolffia columbianaCOMMON WATER MEAL 

Citation:

MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. May 24, 2017. http://michiganflora.net/genus.aspx?id=Wolffia.