POPLAR OF ECONOMIC PLANTS. 333 



Pondweed, Choke {Anacharis Alsinastrwn), an aquatic of 

 the Frogbit family (Hydrochariclacere), with lea^'es like chick- 

 weed, arranged three in a whorl on long, slender, floating stems, 

 wdiich are densely matted together. It is a native of North 

 America, and about forty years ago made its appearance in 

 this country, spreading so rapidly that it was feared it would 

 choke up the canals, and so prevent navigation. It, however, 

 forms such dense masses that it ultimately becomes exhausted, 

 and dies out. 



Pool-rush. {See Bulrush.) 



Poonga Oil, a name in India given to the oil expressed from 

 Pongamia glcibra, a tree of the Bean family (Leguminosa^), very 

 generally diffused throughout Southern India, Malacca, Indian 

 Archipelago, Southern China, ISTorth Australia, and Fiji Islands. 

 It is a wing-leaved, middle-sized tree ; its fruit (pod) from one 

 to one and a half inches long and one inch broad, flattened, 

 roundish, thick, and hard, containing a thick, kidney-shaped 

 seed, which yields an oil, used by the common people for 

 burning in lamps ; and it is also used in medicine. It is of a 

 yellowish-brown colour, and congeals at a temperature below 

 60° Fahr. 



Poplar [Populus), the name of a genus of trees of the Willow 

 family (Salicaccce), of which there are a considerable number of 

 species widely distributed, chiefly throughout the north temperate 

 zone. About 15 sjDecies are recorded as being cultivated in 

 this country, of which tlie following are the principal, the four 

 first being natives : — P. alha, the great White Poplar, or Abele 

 tree, and P. canescens, the Grey Poplar. They are common 

 in other parts of Europe and in the Caucasus and Persia. The 

 timber of P. alha is soft, and but little used, except for toys and 

 such work ; that of P. canesccns is harder, and is used by mill- 

 wrights, and being light is used for the handles of brooms, etc. 

 P. nigra, the Black Poplar, is of rapid growth, attaining a 

 height of 30 to 40 feet in ten years ; the wood is yellow, fibrous, 

 and soft. P. tremula, the Trembling Poplar, or Aspen, is an 

 erect, slender-branching, somewhat pyramidal deciduous tree, 



