4 Reafons for fuppofing that 
of heat are, alfo, requifite for its growth and flow- 
ering. Add to this, that the temperature of the 
bottom of a lake is very different in different parts; 
becaufe, as the depth of the water increafes, its heat 
becomes lefs variable, being always nearly equal to 
the annual mean of the country: but the tempera- 
ture of parts where the depth is inconfiderable is 
nearly as inconftant as that of the air. Such plants, 
therefore, as demand a degree of warmth nearly 
equal to that of the Atmofphere in fummer, are 
_ always found in fhallow places; fuch as are of a colder 
conftitution fix their abode in deeper regions; but 
if any part of a lake be colder than the conftitution 
of the coldeft plant produced in it will bear, that 
part is neceffarily barren.—Aquatic vegetables, at 
certain periods, lofe their vegetative powers, in com- 
mon with thofe that grow on land: But, while the 
latter are converted to earth, and afford additional 
fertility to the foil that raifed them; the former 
preferve their figure, and, in part, retain their 
texture for ages; for water poffeffes an antifeptic 
virtue, that prevents the decompofition of vegetable 
fubftances immerfed in it. When Julius Cefar 
invaded Britain, the natives fortified a ford in the 
Thames, by driving pointed ftakes into the bed of 
the river, with a view of retarding the progrefs of 
the Roman Army. A number of thefe ftakes were 
pteferved undecayed, undoubtedly by their fituation, 
in the time of Rapin the hiftorian, who wrote 
about the beginning of this century, To account for 
this 
