FERNS. 71 
rudiment of a stem exists underground in what is 
called a rhizome, from which 
the fronds shoot out, in 
the same manner as the 
leaves spring from the buds 
of other plants ; these fronds 
have a strong midrib which 
is commonly called a stalk. 
There are said to be between 
two and three thousand va- 
rieties of Ferns; some of 
them, in the tropics, attain 
the enormous height of 
thirty feet. Their growth is 
extremely interesting, the 
fronds opening in a peculiar 
manner, unwinding them- 
selves, as it were, from a round ball. The secd- 
vessels are placed on the back of the fronds in little 
spots or bunches, and the seed is so fine as to be only 
perceptible under the microscope. Ferns thrive best 
in moist and warm situations; if grown under a glass 
vessel which will confine the moisture, they form a 
beautiful and interesting parlor ornament. 
