44 ALTERNATING GENERATIONS 
fluctuations of balance of the two generations of the antithetic alternation 
involved in the upward progress of plant-form. 
The gametophyte was at first the predominant feature, and there is 
good reason, as we shall see later, to believe that it was the originally 
pre-existent phase. It was an independent, self-nourishing organism, with 
unlimited apical growth, and is seen in the Bryophytes either in 
the thalloid form, or developed as a more elaborate leafy plant. In 
the Mosses, and in the leafy Liverworts the 
sexual generation reached its morphological 
climax. But nevertheless in the relative 
simplicity of its tissues, and in the absence 
of an internal ventilating system, it remained, 
as its method of sexuality proclaims it to 
be, at best only an imperfect adaptation 
to growth under conditions of subaerial 
Ne 
/ pA sli exposure. In the homosporous Pterido- 
ee lps e phytes, though there is in Lycopodium, and 
ek in ee ; also in Eguzse¢um, some indication of lateral 
an Gil 1 i appendages, the gametophyte is thalloid, but 
f i | il A it still shows its physiological independence, 
cha L i / while there may be a brief and ill-defined 
apical growth. Nevertheless, in the Pterido- 
phytes the gametophyte as a rule bears the 
stamp of a temporary phase in the cycle 
rather than that of a permanent organism: 
but this becomes much more pronounced 
in the heterosporous forms: in these the 
independent, self-nutritive existence is lost, 
and the prothallus is without localised apical 
Ovary of Polygonum convolvulus during growth : the male gametophyte becomes little 
fertilisation. _/s, stalk-like base of ovary ; a ‘ 
Ju, faniculus ; cha, chalaza; nu, nucellus; more physiologically than a means of pro- 
mi, micropyle ; iz, inner, 7e, outer in- : . a 
tegument; ¢, embryo-sac; ¢&, nucleus of ducing spermatozoids: the female is at first 
sulproe ee een peclles a producer of ova, and later it is simply a 
grains ey Polenrtubes. x48. (After means of nourishing the embryo at second 
hand from the plant on which it is depen- 
dent. The morphological reduction which follows the heterosporous state 
is clear enough in Se/agine//a and in the Pine, and it reaches its climax in 
the Higher Flowering Plants, where the gametophyte is found to have 
dwindled away to an exiguous residuum of a few ill-defined cells, with 
virtually no vegetative characters at all. The whole story indicates the 
eclipse of the generation which appears to have been originally the pre- 
dominant partner in the life-cycle. 
The sporophyte, on the other hand, has a complementary story. It is 
seen in the simplest Bryophytes as an ephemeral, spherical body, without 
distinct apex or base, and no vegetative system except a temporary 
Fic. 30. 
