STRUCTURE AND FUNCTIONS OF FERNS. 23 
In the greater part of the British species, the sori 
are covered during the earlier period of their growth 
with a thin membranous covering, which is called an 
indusium. Some ferns possess this organ, which is 
very evident when they are young. It is, however, 
cast off as the sori attain maturity. The presence 
or absence of this indusium is looked upon as an 
important point in the economy of ferns, and they 
are divided according to this fact. 
Although the spore is often called the representa¬ 
tive of the seed, this little structureless body truly 
represents the bud in higher plants. If we watch 
the development of the spore, we shall find that it 
does not directly grow up into a fern, but that, after 
having attached itself to a damp piece of earth or 
rock, it begins to expand into the form of a green 
membrane. This green membranous mass, on account 
of its resemblance to the fronds of the common Mar- 
chantia, , has been called the Marchantia-like expan¬ 
sion of the fern. It is also called th <z prothallus. On 
the surface of this little body, there appear in the 
course of time two sets of organs. The one called 
pistillidia, containing in their interior little ovoid bodies, 
which are the representatives of the ovules or seed- 
buds of the higher plants. The other organs are 
called archegonia : they contain little movable worm¬ 
like bodies, called spermatazoids , and which are found 
in many of the lower forms of plants. These bodies 
