PTERIDOPHYTA 
395 
Development of sporophyte. The fertilized egg develops at 
once into a sporophyte (Fig. 436). During the early stages 
of its growth the sporophyte is dependent on the prothallus 
for nourishment. The fertilized egg di- 
vides into four segments, one of which 
develops into a foot, which absorbs 
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Fie. 441. Tip of a branch 
of Hquisetum hiemale end- 
ing in a strobilus. (x 1) 
nourishment from the prothallus. 
young embryo, how- 
ever, soon produces 
a stem, roots, and 
leaves, and thus be- 
comes independent 
of the gametophyte. 
After such develop- 
ment has occurred, 
the gametophyte 
dies and the sporo- 
phyte continues to 
live independently. 
Thus, in a typical 
fern (Fig. 438) the 
sporophyte and the 
gametophyte are in- 
dependent plants. 
Alternation of generations. As in the bry- 
ophyta, the cells of the gametophyte contain 
an 2, or single, number of chromosomes, and 
those of the sporophyte a 22, or double, num- 
ber. Again, as in the Bryophyta, the spores 
are formed in tetrads (groups of four) by two 
successive divisions of each spore mother cell. The reduction in 
the number of chromosomes occurs in the first of these two divi- 
sions. The double number is restored when an egg is fertilized by a 
spermatozoid. Therefore the ferns resemble the bryophytes in hav- 
ing an alternation of generations, consisting of a gametophyte with 
an « number of chromosomes and a sporephyte with a 2 2 number. 
The 

Fic. 442. 
reflexum, 
Lycopodium 
with sporo- 
phylls scattered over 
the stem. (X 4) 
