ALTERNATION OF GENERATIONS 



of the ripe seeds already described (Chapter XIX ), account for the 

 Seed-Plants becoming the chief constituent of Land-Vegetation. 

 The disabilities of the gametophyte for land-life have been led. 



In the more adaptive sporophyte the most vulnerable points in I 

 cycle of life, viz. the period of fertilisation, and the first stages of 

 development of the embryo are effectively protected. Thus the sporo- 

 phyte has become virtually the Plant of the Land, and the gametophyte a 

 mere vestige. 



A strong antithesis has been drawn between the relative failure of 

 the gametophyte and the ultimate triumph of the sporophyte in sub- 

 aerial life. The two generations differ normally in chromosome- 

 number. This seems to suggest that a higher potentiality and ini- 

 tiative in variation lies with the diploid state. It may not be possible 

 to lay it down as a general proposition that a double number of 

 chromosomes is an index of greater power of adaptability ; but it 

 is a significant fact that the highest somatic evolution both in Animals 

 and Plants has been attained by diploid, not by haploid tissues. 

 The gametophyte is the sexual generation, and the sexual organs 

 are borne by it. In this it stands in strong antithesis to the sporophyte, 

 which is a non-sexual or neutral generation, by nature and by origin. 

 But the steps in the obliteration of the gametophyte, and in the evolu- 

 tion of the seed have been accompanied by a tendency for sexual 

 differentiation to encroach more and more on the morphology of the 

 sporophyte generation. This will be apparent on comparison of the 

 life-cycle of a homosporous Fern (Fig. 400, p. 506) with that of a 

 Flowering Plant (Fig. 257, p. 335). The final culmination of this 

 found in those Seed-Plants which are dioecious, such as the Willow, 

 or the Yew (Fig. 258, p. 336). In these some individuals bear only 

 staminate, others only pistillate flowers, and the plants are thus ranked 

 as " male " or " female," though in point of fact they represent the 

 neutral generation. The end result is thus seemingly a paralhl 

 between the Higher Plants and the Higher Animals . 1 xuality. 



In both the individual appears to be either " male " or " female." But 

 this similarity is superficial rather than real, for it has been attained 

 along quite distinct evolutionary trends in the two Kingdoms. In the 

 Higher Animals there is a true sex-difference between individual.-, 

 the one producing male, the other female gametes. In the Flowering 

 Plants the individual is the neutral sporophyte, which does not itself 

 produce gametes. But in the course of Descent certain distinctr 

 features relating to sex have become increasingly evident in the 

 sporophyte or neutral generation. Accordingly the Flowering Plant 



