. Bulletin. Natural history; Natural history. May. 1943 Bellrose & Anderson: Duck Food Plants 427. Fig. 11.—Longleaf pondweed {Potamogeton americanus) ranks eleventh among Illinois River valley plants as a source of duck food. It ranks above sago pondweed apparently because it produces more seed. value, but below the nutgrass group. Seed production studies (Low & Bell- rose ms.) revealed that in 1941 teal grass produced about half as much seed as Cyperus erythrorhizos and almost as much as did duck potato per area unit. Duck potato, Sagittaria latifolia, fig. 9, has an index value, tab
. Bulletin. Natural history; Natural history. May. 1943 Bellrose & Anderson: Duck Food Plants 427. Fig. 11.—Longleaf pondweed {Potamogeton americanus) ranks eleventh among Illinois River valley plants as a source of duck food. It ranks above sago pondweed apparently because it produces more seed. value, but below the nutgrass group. Seed production studies (Low & Bell- rose ms.) revealed that in 1941 teal grass produced about half as much seed as Cyperus erythrorhizos and almost as much as did duck potato per area unit. Duck potato, Sagittaria latifolia, fig. 9, has an index value, table 3, about two- fitths as great as that of giant bur-reed. Incidentally, in 1941, bur-reed pro- duced about two-fifths more seed per unit of area than did duck potato. For the most part, the large tubers ot duck potato, often a foot or more under- ground, are not available as duck food. However, at times canvasbacks and ringnecks have succeeded in obtaining numerous tubers, which considerably raised the value of duck potato for cer- tain areas. Marsh smartweed, Polygonum Muh- lenber^ii, fig. 10, has in table 3 an index value of , which places it slightly below duck potato. Beds of this spe- cies will not produce seed when growing out of water; optimum production oc- curs in water 12 to 18 inches deep. In several areas, beds produced no seed be- cause of a lack of water. This species would rank somewhat higher if all the beds that failed to produce seed were ex- cluded from the calculations. Longleaf pondweed, Potamogeton afuericanuSy fig. 11, ranks below marsh smartweed in index value, table 3, de- spite the fact it outproduces the latter in seed yield by a wide margin. This may indicate that, because of the greater depth of water at which the plants grow, seeds of aquatic plants are less accessi- ble to dabbling ducks than are those of marsh plants. Buttonbush, Cephalanthus occiden- talism seldom grows in the areas that were mapped, occurring usually as a part of, or wi
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