_—— Se 
FERN STRUCTURE 255 
structures consisting of an outer layer of cells surrounding 
a central mass of small cells, each of which produces 
a sperm. When mature, the antherids rupture and 
permit the escape of the spiral multiciliated sperms 
which swim with a rotary motion. 
452. The archegones are flask-shaped organs sunken 
into the tissues of the plant. At first 
the neck is closed, but at maturity it his : 
opens down to the egg. Fertilization 
takes place in water (after rains or 
heavy dews), the sperms swimming 
to and down the neck of the arche- Fie. 131.—Fern arch- 
gone, where one of them unites with sperm. resign 
the egg. 
453. Sporophyte. After fertilization the egg divides 
again and again, soon producing a solid stem from which 
a root springs at one end, while from the other the leaves 
arise. The latter are at first small and quite simple in 
structure, but those formed later are larger and more 
and more complex in structure, until finally the full form 
is reached, and still later the full 
size. The stem, bearing leaves 
and roots, constitutes the sporo- 
phyte, which is sharply contrasted 
with the gametophyte in structure, 
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Fic. 132.—Development of size, and duration, the latter being 
f rophyte. 
iio short-lived, small, and of simple 
structure, while the former is long-lived, often of large 
size, and of great complexity of structure. On this 
plant the spores are eventually produced which on 
germination give rise to gametophytes like those with 
which we started, thus completing the round of life. In 
most Ferns the spores are of one kind, only (isospores), 
but in a few they are of two kinds (heterospores) in 
