240 
PHARMACEUTICAL BOTANY 
small brown patches each of which is called a sorus, and usually 
covered by a membrane called the indusium. Each sorus consists 
of a number of sporangia (spore cases) developed from epidermal 
cells. In some ferns the entire leaf becomes a spore-bearing organ 
(sporophyll) . Most sporangia have a row of cells around the margin, 
the whole being called the annulus. Each cell of the annulus has a 
U-shaped thickened cell wall. Water is present within these cells 
and when it evaporates it pulls the cell walls together, straightening 
the ring and tearing open the weak side. The annulus then recoils 
Fig. i 15 . — Cyrlomium falcatum. Under (dorsal) surface of a portion of a 
sporophyll, showing the numerous sori on the pinnae. {Gager.) 
and hurls the spores out of the sporangium. Upon coming into con- 
tact with damp earth each spore germinates, producing a green sep- 
tate filament called a protonema. This later becomes a green heart- 
shaped body called a prothallus. It develops on its under surface 
antheridia or male organs and archegonia or female organs as well 
as numerous rhizoids. Within the antheridia are developed motile 
sperms , while ova are produced within the archegonia. The many 
ciliate sperms escape from the antheridia of one prothallus during a 
wet season, and, moving through the water, are drawn by a chemo- 
tactic influence to the archegonia of another prothallus, pass down 
the neck canals of these and fuse with the ova, fertilizing them. 
The fertilized egg or oospore divides and redivides and sson becomes 
