( *9<> ) 
be explained. The plants comprifed under the 
defcription of alga*, or flags, fcarcely admit of a 
diftin&ion of root, item, or leaf ; much lefs are 
their flowers fufficiently obvious to admit of a 
definition of their parts, though, by the fituation 
of their fuppofed flowers, or feeds, the genera 
are diftinguiflied, or fbmetimes by the re- 
femblance of the whole plant to fome other 
fubftance with which we are familiar in the 
economy of nature. This tribe of plants is 
of great importance, as they frequently afford 
the firfl: foundation, from which other plants 
draw nourifhment. One fpecies of byffus, and 
feveral fpecies of lichen, fix upon the bareft 
rocks, and are fupported by what llender 
fupply the air and rains afford them. Dr. 
Smith, in his tour on the continent, in the 
years 1786 and 1787, found near Mount 
Vefuvius, on a torrent of lava, which iflued in 
1 77 1, the lichen pafchalis, which covered it 
mod copioufly, and had the appearance of 
hoar froft, with no other plant near it. The 
lichen pafchalis is peculiarly fitted for the be- 
ginning of vegetation on the hard furface of 
iva, from it's fhrubby figure, and flender 
Qts ; in the fame manner, the thread-form 
:iens infinuate their roots into crevices in 
the 
