ALTERNATION OF GENERATIONS 487 



spermatozoid by the non-motile gamete delivered by the pollen-tube 

 (Figs. 213, 214). Thus the most critical point of each life-cycle, viz. 

 fertilisation, was finally adapted to Life on Land. And so by a 

 series of steps the gametophyte is reduced, altered, and in some cases 

 almost obliterated. It has paid the penalty of its inability to adapt 

 itself thoroughly to sub-aerial conditions. The climax of the game- 

 tophyte on land is attained in the homosporous Mosses and the leafy 

 Liverworts. It appears of independent, though limited growth in 

 the homosporous Ferns. But with heterospory it fades into insignifi- 

 cance, and in the Higher Seed-Plants it survives as a mere relic. 



RISE OF THE SPOROPHYTE. 



The sporophyte, on the other hand, is seen in the ascendant in the 

 progressive series of Land-living Plants. By adaptation of form and 

 structure it has met, in its highest terms successfully, the require- 

 ments for mechanical strength, for protection under drought, for 

 exposure of a large surface for Photo-Synthesis, and for ventilation 

 of extensive nutritional tissues. In all these respects it is the 

 superior of the gametophyte, and perhaps the structural feature 

 that has contributed most to its supremacy is the ventilation- 

 system of intercellular spaces, controlled by stomata at their 

 exits to the open air. This differentiates the sporophyte from the 

 gametophyte more clearly than any other structural character, and 

 stamps it as adapted to sub-aerial life. The end of its vegetative 

 existence is the formation of spores (carpaspores} . The more numerous 

 these are (other things being equal) the better the chance of sur- 

 vival of the species, and of its spread. The vegetative development 

 may be regarded as a means to that end, and in homosporous forms 

 its nutritive capacity imposes the only limit on spore-numbers. The 

 dispersal of the spores is dependent in primitive land-forms upon a dry 

 atmosphere. This is in strong antithesis to the necessity for external 

 fluid water for fertilisation, which is the end of their gametophyte. 

 The apparent steps in elaboration of the sporophyte, and in its speciali- 

 sation for the certain establishment of germs under sub-aerial con- 

 ditions may now be briefly sketched, the illustrations being drawn 

 as far as possible from organisms described in previous chapters. 



The origin of the sporophyte in the Archegoniatae is quite problematical, 

 since no certain ancestry is known for them. It has been suggested on the 

 one hand that the dependence of the sporophyte upon the gametophyte is 

 a true indication of its derivative origin. In that case the sporophyte may 



