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 

 exposure. In the homosporous Pterido- 

 phytes, though there is in Lycopodium, and 

 also in Equisetum, some indication of lateral 

 appendages, the gametophyte is thalloid, but 

 it still shows its physiological independence, 

 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 

 growth : the male gametophyte becomes little 

 more physiologically than a means of pro- 

 ducing spermatozoids : the female is at first 

 a producer of ova, and later it is simply a 

 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 Selaginella 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 



FIG. 30. 



Ovary of Polygenum convolvulus during 

 fertilisation, fs, stalk-like base of ovary ; 

 fn, funiculus ; cha, chalaza ; nu, nucellus ; 

 mi, micropyle ; zV, inner, ie, outer in- 

 tegument ; e, embryo-sac ; ek, nucleus of 

 embryo-sac ; ei, egg-apparatus ; an, anti- 

 podal cells ; g, style ; n, stigma ; p, pollen- 

 rains ; ps, pollen-tubes. X48. (After 

 trasburger.) 



