Aquatic Seed Plants 153 



the larger water meadows within our flora. They have 

 alternate leaves and slender flexuous stems that are 

 often incrusted with lime. 



There are evergreen species among the Potamogetons, 

 and other species that die down in late summer. There 

 are broad leaved and narrow leaved species. There 

 are a few, like the familiar Potamogeton natans whose 

 uppermost leaves float fiat upon the surface, but 

 the more important members of the genus live wholly 

 submerged. Tho seed-plants, they mainly reproduce 

 vegetatively, by specialized reproductive buds that are 

 developed in the growing season, and are equipped with 

 stored starch and other food reserves, fitting them when 

 detached for rapid growth in new situations. These 

 reproductive parts are developed in some species as 

 tuberous thickenings of underground parts; in others 

 as burr-like clusters of thickened apical buds ; and in 

 still others they are mere thickenings of detachable 

 twigs. 



The Potamogetons enter largely into the diet of wild 

 ducks and aquatic rodents and other lesser aquatic 

 herbivores. They are as important for forage in the 

 water as grasses are on land. 



Other naiads are Nais (fig. 85) and Zannichellia. 



Eel-grass {Vallisneria) is commonly mixed with the 

 pond weeds in lake borders and water meadows. 

 Eel-grass is apparently stemless and has long, flat, 

 flexuous, translucent, ribbonlike leaves, by which it 

 is easily recognized. The duckweeds (Lemnaceae, figs. 

 61 and 62) are peculiar free-floating forms in w^hich the 

 plant body is a small fiat thallus, that drifts about freely 

 on the surface in sheltered coves, mingled with such 

 Hverworts as Ricciocarpus, with such fernworts as 

 Azolla, with seeds, eel-grass flowers, and other flotsam. 

 There are definite upper and lower surfaces to the thal- 

 lus with pendant roots beneath hanging free in the 



