PHYLETIC DELAY IN REDUCTION 77 



reduction, follows on nuclear fusion, while in Rhopalodia these events 

 precede nuclear fusion. Such cases seem to point to a probability that 

 the problem of reduction was solved independently and in different ways 

 in different lines of descent. 



Nevertheless the term sporophyte has been adopted as applicable 

 collectively to the non-sexual phase which intervenes between sexual fusion 

 and reduction in those plants in which it occurs. But, following the above 

 reasoning, it must not be understood to convey necessarily any community 

 of descent for all the bodies which it covers. It seems probable that the 

 establishment of the sporophyte, whether by a process of intercalation or 

 otherwise, has taken place independently in several distinct phyla : thus 

 the sporophyte-stage in them, though in some more lax sense it may be 

 styled " homologous," is not to be held as " homogenetic " ; nevertheless, 

 useful analogies may be drawn between the corresponding phases in 

 distinct phyletic lines. 



But, on the other hand, comparison within groups that are held to be 

 akin gives strong reason for recognising that there has been a shifting of 

 position of the event of chromosome-reduction in certain lines of descent, 

 and that the balance of the generations has thus been altered in the 

 evolutionary course. For instance, it seems probable that in the Uredineae 

 there has been a deferring of the event of reduction after sexuality, with the 

 result that the binuclear phase has attained considerable dimensions ; the 

 same seems probable also for the Ascomycetous Fungi, though along a quite 

 distinct line. A similar intercalation has been suggested in the Florideae. 

 Such conclusions can only become cogent when the cytological details are 

 known in a large number of related forms. But the most familiar, and at 

 the same time the most prominent and permanent example is that of the 

 Archegoniatae : in these there is a strong comparative basis for the belief 

 that the sporophyte stage has been intercalated, or in any case greatly 

 extended, in consequence of the deferring of the event of chromosome- 

 reduction. It may be a question whether the post-sexual stage in the 

 life-history of certain green Algae represents any phyletic predecessor of the 

 sporophyte of the Archegoniatae : it is quite probable that it did not. But 

 this much is clear, that it occupies the same position in the life-cycle, and 

 it gives at least the suggestion how the Archegoniate sporophyte may 

 have originated. According to the antithetic theory as applied to the 

 Archegoniatae, the complications of post-sexual nuclear reduction, involv- 

 ing, as they are seen to do, at least four nucleated cells, supplied the 

 starting-point for a diploid somatic expansion. That is the theory which 

 is adopted here as reflecting the probable mode of origin of the alternation 

 in the Archegoniate series. But it is only right to acknowledge that it 

 is not fully demonstrated either by the cytological facts, or by comparison 

 with the alternation in the Thallophytes. The latter can only supply 

 suggestive analogies so long as the actual phyletic origin of the Archegoniatae 

 remains as obscure as it now is. It becomes, accordingly, an object of all 



