1054 REPORT— 1898. 



a stage resembling the Bryophyte sporogonium, but that the origin of this spcond 

 generation in the ancestral Algce was homologous. But the homologous theory 

 does not necessarily assume the existence of the sporogonial stage. The sporo- 

 phyte of the Vascular Cryptogams may have had an independent origin from that 

 of the Bryophyta, and have resulted from the modification of individuals, which 

 ■were never reduced to the condition of a fruit body. 



As to the circumstances which led to alternation of generations, the two theories 

 are in essential agreement. We owe to Professor Bower the general statement, 

 which must serve as the starting point of any explanation, that the origin of the 

 alternation may be correlated with a change of habit from aquatic to sub-aerial 

 life. This holds whether the second generation is considered to be homologous 

 with the first, or to be the result of interpolation. On the latter view, which is 

 that elaborated by Professor Bower, the importance of the drier conditions of life 

 is sought in the prevention of repeated acts of fertilisation. It would thus have 

 been an advantage to the organism to produce many individuals as the result of 

 one sexual act, and this is seen to be effected with increasing perfection as we pass 

 from the simpler to the more complex Bryophyte sporogonia, and from these to the 

 Pteridophyta. The same change of environment may, however, have initiated the 

 modification of individuals, which were originally potential sexual plants, into 

 spore-bearing forms. We shall return to this when discussing apogamy. 



We have seen that the facts of morphology do not of themselves indicate 

 decisively which theory is the correct one. The reasons which render one or the 

 other view the more probable are bound up with the more general question of the 

 course of descent in the vegetable kingdom. The question of the relationship 

 between the main groups of plants is a very complex one. All that we need do 

 here, however, is to recognise the existence of several alternative views, and the 

 bearing of these on the two theories of alternation. The indications of alternation 

 in the Thallophytes may be first referred to. These seem closely comparable to 

 the simplest Liverwort sporogonia, but it has not been suggested that any direct 

 relationship exists in any case. The existence of these rudimentary sporophytes 

 in various Green AAffie, in Cystopus, and in an analogous, though distinct form in 

 Ascomycetes and Floridete, is indeed strongly suggestive of their independent 

 origin in the Thallophytes of the present day, and justifies us in considering it 

 probable that similar developments may have occurred in the ancestral Algal forms 

 ftom which the Archegoniates arose. But the further recognition of the possibility 

 that the origin of the Archegoniatse may have been polyphyletic, and in particular 

 that the Vascular Cryptogams may have had a line of descent from Thallophytes 

 perfectly distinct from that of the Bryophyta, has a much more important bearing 

 on the nature of alternation. The gap between Bryophytes and Pteridophytes ia 

 wide, and on this view would be an essentially natural one ; any attempt to bridge 

 it would involve misleading conclusions. I do not wish to enter into the question 

 of the polyphyletic origin of archegoniate plants further than to show that its 

 possibility must be borne in mind in considering the nature of alternation. It maybe 

 pointed out, however, that such a view would appear to follow naturally from the sup- 

 position that the origin of the sporophyte was correlated with the spread of aquatic 

 organisms to the land. It may be considered probable that a number of organisms 

 in different places would have undergone more or less similar modifications. The 

 homologies which exist between the spore-bearing generations of Mosses and Ferns 

 are no less possible results of homoplastic developments than others in favour of 

 which direct evidence exists. If the origin of the Pteridophyta has not been from 

 the Bryophyta, the comparison between the sporogonia of the latter and the simpler 

 sporophytes of the Vascular Cryptogams would lose much of its weight, since the 

 two may have proceeded, as Qoebel suggested, on distinct lines from the beginning. 

 It is therefore advisable to ascertain if any evidence exists which may indicate how 

 the Vascular Cryptogams could have been derived directly from Algal forms. 

 Something of the kind, as we shall see, may possibly be afforded by the facts of 

 apogamy. 



ISo far we have seen no reason to regard the nature of alternation and the views 

 on descent which underlie it as anything but open questions. There are, however, 



