FERTILIZATION. 19 
spiral spring. These are termed the sporangia, or spore- 
cases. When the sporange is ripe this elastic spiral spring 
breaks, scattering the contents (spores) in a thousand direc- 
tions, as the pollen in flowering plants. A great many ferns 
have a delicate membranous covering for the little patches; 
this is the indusium or involucre. ‘The form of the indu- 
sium is a characteristic feature in the classification of ferns. 
Sometimes it is linear, sometimes round, and sometimes 
fringed. In describing the several species this will be more 
particularly mentioned. Some ferns, as the common Poly- 
pody and Beech Ferns, are without any indusium, but these 
are exceptional cases. 
When the spore is set free it alights on some con- 
venient place, and germination, or rather budding, then 
commences. Some spores will germinate in three or four 
days, while others will require several weeks. The germi- 
nation of a fern-spore is very different from that of the seed 
of a flowering plant or of an ordinary tree. With proper 
heat and moisture a young oak will be developed from an 
acorn planted in the soil, but the true fern requires two 
generations for its development. The minute, almost 
invisible spore bursts, its contents bud and form new cells; 
these cells continue to enlarge, and eventually become a 
flattish, heart-shaped body, technically called the prothallus. 
The prothallus may be compared to a flower, since it con- 
tains the fertilizing organs, antheridia, the male, and arche- 
gonia, the female. The development of the spore, in its 
various stages, is represented in Plate II. 
A number of small roots are formed beneath the prothal- 
lus. These roots are mere hair-like bodies, each consisting 
of a single tube, and quite transparent. After the archego- 
nium is fertilized a bud is formed, and from this bud the 
tender and delicate frond of the fern is unfolded. 
