374 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1940 



land except where the stand is thin on the margin of the stream. 

 Here straggling plants of Polygonum sagittatwm and species of Sagit- 

 taria and Bidens are sometimes present. On the land side of this 

 zone the dominant species is Typha latifoUa^ which under conditions 

 of less water and greater silt deposit may displace Zizania aquatica. 

 The Typha area itself is soon invaded by Peltandra virginica, Iris 

 versicolor, and several species of Hibiscus and Polygonum. As the 

 marsh becomes more elevated and drier, shrubs and finally trees be- 

 come dominant. 



In the lakes of Minnesota and Wisconsin, just beyond the zone con- 

 taining such submerged plants as the Potamogeton species and Val- 

 lisneria spiralis, the bottom is often thickly covered with plants 

 having lime-encrusted leaves and stems called stoneworts. The 

 Zizania area is usually bordered on the outside margin by several 

 species of Scirpus, never by Pontederia cordata as along the Potomac 

 River, and on the land side by Carex, though Typha is often present. 



During early April in the vicinity of Washington, D. C., the seed- 

 lings of Zizania aquatica have already emerged from the muck- 

 covered flats and are strong enough to stand erect when the tide is 

 out, like seedlings in any grainfield. Within a week or 10 days after 

 emergence the young plants have three leaves. Growth is slow at 

 first largely because of the low temperature of soil and water and in- 

 termittent sunshine. During these days they are strengthening their 

 grip upon Mother Earth, for good anchorage in the soil at this time 

 reduces the hazard of being washed away by tides, especially the ebb 

 flow. The protection of the marginal plants that they have in later 

 life is lacking now. The plants become more robust as spring ad- 

 vances, and by June, where the stand is good, the gi'owth gives 

 the marsh the appearance of a low meadow. In another month the 

 stems that have been concealed by the enveloping leaf sheaths during 

 this vegetative growth begin to give some evidence of their existence. 



The part of the stem embraced by the sheath of the last leaf is 

 growing rapidly now and being confined within a narrow space dis- 

 tends the sheath into a spindlelike appearance. Ten days after the 

 inception of this condition is perceptible, the panicle, which is the 

 terminal of the stem, begins to emerge. The panicle emerges slowly, 

 often requiring 7 days to free itself from this cover. It carries on 

 its distal end the female flowers that bloom almost immediately upon 

 emergence. They, at once, are receptive to wind-borne pollen from 

 nearby or distant plants, for the male flowers of the same plant are 

 still within the leaf sheath. 



The male flowers on the lower part of the panicle hang like elon- 

 gated bells, which open a few days later, revealing six yellow bodies 

 like so many clappei*s, which are filled with pollen that is soon dis- 



