THE PTERIDOPHYTES 433 
like portion called a stipe which is extended into a lamina usually 
showing a forked venation. Some ferns possess laminae which 
are lobed, each lobe being called a pinna. If a pinna be further 
divided, its divisions are called pinnules. In some ferns the 
pinnules are further subdivided into secondary pinnules, as in the 
“brake? or “bracken,” Pterts aquilina. The unfolding of a 
frond is circinate and it increases in length by apical growth. 
On the under surface or on the margin of the lamina, pinnz, or 
pinnules may be seen small brown patches each of which is called 
a SOrUs, and usually covered by a protective membrane called the 
indusium. Each sorus consists of a number of sporangia (spore 
cases) developed from epidermal cells. In some ferns the entire 
Ieaf becomes a_spore-bearing organ (sporophyll). Many 
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 
and hurls the spores out of the sporangium. Upon coming into 
contact with damp earth, each spore germinates, producing a 
green septate 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, many ciliated 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 chemotaxic 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 odspore divides and redivides and soon becomes differ- 
entiated into a stem, first leaf, root, and foot. The foot obtains 
nourishment from the prothallus until the root grows into the soil, 
when it atrophies, and the sporophyte becomes independent. 
Unequal growth and division of labor continue until a highly 
differentiated sporophyte results, the mature “fern plant.” 
In some species of ferns as the Cinnamon Fern (Osmunda 
cinnamomea) and the Sensitive Fern (Onoclea sensibilis), two kinds 
