STRUCTURE OF FERNS. 
21 
of full-grown plants. Though these forms of production 
cannot be called common, in most species when 
produced they are not difficult to raise from the bud to 
the perfect fern. It is a remarkable character of ferns 
and club-mosses that, though impregnated but once, 
they yet continue to yield fertile spores year after year, 
sometimes, even to the age of one hundred years. 
c 
Fig. 12. 
The structure of ferns is almost as interesting as their 
reproductive system. 
One of the most remarkable vegetable tissues, called 
from its peculiar markings, which somewhat resemble 
the rounds of a ladder, is scalariform tissue (fig. 12). 
