GROWTH OF FERNS. 
15 
Having thus briefly mentioned a few of the most 
common ferns, we must now consider the growth and 
structure of a fern, taking as our typical specimen the 
Bracken or Brake Fern (Pteris aquilind). This can be 
commonly found in nearly all parts of Great Britain. 
A good fertile frond may be readily known by the back 
of the frond having dark-coloured bodies (sort) near the 
edge of each division ( pinnce ). These sori when ripe dis¬ 
charge round spore-cases called sporangia. In most ferns 
these spore-cases are surrounded by an elastic ring 
{annulus), fig. 9, c. When the ripe dryness of the sori 
causes the membrane ( indusium ) which covers the spor¬ 
angia (fig. 10, d) to split or shrink up, these spore-cases 
fall out, and almost immediately, unless they should be 
