May, 1943 
to changes in seed production resulting 
from changes in environment. Such 
changes are well illustrated by water 
hemp at Clear Lake. In 1940, because 
of water level changes that reduced seed 
yield, water hemp had an index rating 
only half as great as in the previous 
year. Among aquatic plants, giant bur- 
reed, marsh smartweed and sago pond- 
weed are notable as species that vary 
BELLROSE & ANDERSON: Duck Foop PLANTS 
419 
greatly in the yield of seed with vari- 
ances in environmental conditions. 
In some years, plant seeds present in 
abundance may be inaccessible to all or 
some ducks through the absence of 
water or through a depth of water too 
great for dabbling species. In other 
years, seeds of plants not present in 
those years may be available from prev- 
ious years. 
For example, in 1938, be- 
Fig. 1.—Rice cut-grass (Leersia oryzoides), or sawgrass, in most years and in most Illinois 
River valley habitats leads all other uncultivated plants in value as a source of food for ducks. 
Not only is this species valuable for its seed, which it produces in fair abundance, but its root- 
stocks and shoots also furnish food for waterfowl. 
