32 THE LIFE-HISTORY OF A FERN 
Fern-Plant. If the events above detailed recur in regular succession 
there will be seen a regular alternation of two phases of life, or generations : 
of these the one, the prothallus, bears the sexual organs, which contain 
the sexual cells or gametes, and it may accordingly be styled the 
gametophyte: the other, the Fern-Plant, is non-sexual or neutral, and bears 
the sporangia, containing the spores: accordingly it may be styled the 
sporophyte. The study of Ferns at large leads to the conclusion that this 
regular alternation of generations 
Fite is typical for the family. These 
two alternating generations differ 
not only in form, but also in 
their relation to external circum- 
stances, and especially in the 
Ye water-relation. The sporophyte 
“Pays ory? is structurally a land-growing 
; pn plant, with nutritive, mechanical, 
and conducting tissues, and a 
ventilating system: not only is 
ZYGOTE it capable of undergoing free 
SPORE ff exposure to the ordinary atmo- 
rermnstozoy/ Mem spheric conditions, but dryness 
of the air is essential for the 
Asia final end of its existence, viz. 
Archegonuum nag ; 
the distribution of the spores. 
On the other hand, the gameto- 
GAMETOPHYIE phyte is structurally a plant 
ill-fitted for exposure, with un- 
differentiated and_ ill-protected 
tissues, and no __ ventilating 
Gonepiyte* system, while the object of its 
eo existence, viz. the fertilisation 
Fic. 16. carried out by the sexual organs, 
Scheme of life-cycle of a Fern. can only be achieved in the 
presence of external fluid water. 
There is thus a marked difference between these two phases, and their 
sequence may be said to constitute an antithetc alternation. As regards 
the water-relation, the whole life-cycle of the Fern might not inaptly 
be designated as amphibious, since, the one phase is dependent: on 
external fluid water for achieving its object of propagation, while the 
other is independent of it. 
It will be seen in the next chapter that this is not a condition for 
Ferns alone, but that such alternation as is seen here has its parallel in 
many other plants, though with great differences in detail, and especially 
in respect of the balance of size of the two generations. 
