FIELD BOOK OF PONDS AND STREAMS 



beds with their population of little clinging organisms furnish 

 shelter and food for most of the smaller fishes of the 



lake. 



Pondweeds are one of the principal foods of wild ducks. 

 They are so vital to animal life, especially to wild fowl, that 

 ponds are planted with them and a growing literature is 

 concerned with their biology and economic importance. 

 Bullheads, perch, and pickerel hide among them and a large 

 snail population, chiefly of Planorhis, Lymnma, and Physa, 

 gets its living from their leaves. 



Fig. 62. — Pondweeds: i, Potamogeton natans; 2, 

 P. crispus. 



Potamogeton natans. — This is one of the commonest pond- 

 weeds (Fig. 62). Its floating leaves have long stems or peti- 

 oles and oval blades but the submerged ones are incomplete 

 and thread-like, only the petiole being developed. 



Occurrence.— In shallow ponds; frequent. Blooms June- 

 September. Almost cosmopolitan in temperate countries. 



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