CHAPTER XLIX 



SUMMARY OF RESULTS 



A BRIEF Organographic History will now be given of the probable steps 

 that have been taken in the evolution of the Filicales. That history does not 

 purport to be an essay in Taxonomy: but if it has been correctly gathered 

 from the available facts, both of the present and the past, it should itself 

 provide the main outlines of a Systematic Grouping of the Class. The story 

 will necessarily be discontinuous, and there can be no certain beginning: 

 except such as may be reconstructed from the general characteristics of 

 those Filical types which we regard as primitive, and their comparison with 

 those of organisms held as lower in the scale of vegetation. The aim will be 

 to trace the successive steps of their advance and modification, keeping 

 constantly in view the biological aspect of those changes which appear. 



Alternation 



The most general characteristic of all is the alternating life-cycle, for not 

 only is this present in all Ferns, but also in all Archegoniate plants, while 

 it extends in some form or another to all plants that reproduce sexually. 

 This is not the place to discuss its nature or its origin. Here we accept it as 

 an objective fact, which lies at the back of all higher organography, not only 

 of Ferns, but also of land-living plants generally, and of most Algae and 

 Fungi. But whereas in the Thallophytes the events of the cycle appear less 

 stable, and are less definitely related to distinct types of somatic develop- 

 ment, in the Archegoniate plants generally, and in the Ferns in particular, 

 the cycle is defined organographically. Though exceptions are numerous, 

 the normal alternation in Ferns is between the simpler sexual gametophyte 

 and the more complex non-sexual sporophyte. The high degree of stability 

 in the succession, the organisation, and the proportion of those phases is a 

 fundamental feature in the organography of Ferns, as it is of Vascular 

 Plants at large. But in the Algae, and especially in the less highly organ- 

 ised types of them, that stability is less marked: the facts indicate that in 

 this there is a difference between sub-aerial and aquatic vegetation — but it 

 is a difference of degree rather than of kind. 



The reason for this greater stability of the alternating generations in 

 land-living plants may properly be sought in the difference of environment. 

 It is the leading function of the gametophyte to bear gametangia, with 

 syngamy as the final result: each fusion initiating a new sporophyte. Spore- 

 production is the leading function of the sporophyte, each spore initiating a 



