FRESH-WATER PLANTS. 21 



with a greenish slime ? If now a minute por- 

 tion of this matter be taken on a slip of glass, 

 spread out with a drop of clear water, and then 

 placed beneath a good microscope, it will be 

 found to be composed of an assemblage of 

 plants, often exceedingly elegant and beautiful 

 in their structure. The number of species of 

 conferva; and the allied genera is very great, 

 probably not less than six hundred. Their 

 propagation is often exceedingly rapid, perfect 

 plants being often entirely reproduced in from 

 twenty-four to forty-eight hours. Ponds, which 

 have been entirely cleared of conferva one day, 

 have become quite covered by them the next. 



In tropical countries, the confervce are very 

 rare, though not altogether absent. The duck- 

 weed (Lemna) is an extremely common and 

 well-known fresh-water plant ; four species of 

 it inhabit England, two of which have also been 

 found in America and Australia. It is general 

 throughout temperate climates. In tropical 

 countries, the duck-weeds cease, and are re- 

 placed by a larger plant, with flowers and 

 leaves, (Pistia stratiotes,) which covers many of 

 the tropical lakes in incredible quantities. 

 When there have been storms on the Laguna 

 de Bay, the great lake in the island Lucon, 

 these plants are thrown on the coasts, and not 

 only cover the edges of the water, but form 

 heaps several feet high on the shore, and spread 

 a dreadful stench while decaying. The same 

 plant grows abundantly in the water-tanks in 

 Jamaica, and is said, by the acrid properties 



