1052 REPORT— 1898. 



investigation have raised anew the question of the nature of this Alternation of 

 Generations. The subject has been discussed from two very different standpoints in 

 the Presidential Addresses to this Section of the British Association this year and at 

 the Liverpool meeting.^ These expressions of opinion by Dr. Scott and Professor 

 Bower render an introductory paper to this discussion in one sense superfluous. 

 While, however, repetition of much that has been already said is unavoidable, the 

 existence of such diverse views suggests a slightly different treatment of the ques- 

 tion, which may be useful for the purposes of the discussion. Instead of advoca- 

 ting either the theory of antithetic or of homologous alternation, I shall try to 

 present a dissection of the subject ; with this object the main facts known as to 

 alternation of generations will be briefly discussed, and the possible interpretations 

 of them considered. The facts will as far as possible be kept apart from the 

 theoretical views to which they have given rise, and the points on which our know- 

 ledge is deficient will be emphasised rather than minimised. 



The general facts regarding alternation of generations in archegoniate plants 

 can be dismissed very briefly. In all the main groups a definite alternation of 

 a sexual with an asexual generation is found. The latter is normally developed 

 from the fertilised ovum, the former from the spore. The Bryophyta and Pterido- 

 phyta are, however, opposed to one another in the relative complexity attained by 

 the two generations. The sporophyte in the Bryophyta remains dependent on the 

 Moss or Liverwort plant, and has as its main function the production of the spores. 

 It may, however, attain very considerable complexity of structure and possess a Avell 

 developed assimilation tissue. In both Hepaticse and Muscinese very simple sporo- 

 gonia lead on to complex ones in which the sterile tissue of the wall, foot, seta, &c. 

 forms a considerable proportion of the whole structure. The gametophyte, on the 

 other hand, is always independent, and often shows a complicated external form 

 with clearly differentiated stem and leaves. In the Vascular Cryptogams also the 

 gametophyte is always independent, but is of relatively simple form and structure. 

 The sporophyte, which develops from the fertilised ovum, very soon produces roots, 

 and attains independence by the death of the prothallus. It shows a distinction 

 of stem and leaf, is highly organised, and does not develop spores until after a 

 period of vegetative growth. While these points of difference which indicate the 

 great gap between the Bryophyta and Pteridophyta are borne in mind, due weight 

 must be given to the points of agreement. Of these, the similar structure of the 

 sexual organs, the fact that in both the sporophyte is at first dependent on the 

 gametophyte, the presence of stomata and intercellular spaces in the sporophyte, 

 and the similarity in the spore production may be mentioned. A consideration of 

 these facts by themselves indicates no view as to the mode of origin of the two 

 generations. At no stage do the two generations in any Archegoniate closely 

 resemble one another, except in the case of the young plant and the prothallus of 

 Lycopodium cernuum. The deviations from the normal life history, which will be 

 considered later, may somewhat modify this statement. 



We are justified in assuming that the Bryophyta and Pteridophyta arose from 

 ancient Thallophytes ; the study of the life histories of the Algse and Fungi, which 

 exist at present, may accordingly be expected to aid in arriving at probable con- 

 clusions as to the origin of the alternation in archegoniate plants. It is naturally 

 among the green Algse that indications of this sort might be expected, nor are 

 they wanting, though the precise weight to be attached to them is a matter of 

 uncertainty. The higher Fungi and the Red and Brown Algse may for the sake of 

 simplicity be left on one side with the remark that in Ascomycetes and Floridese 

 we see a development which presents analogies with the alternation in Archegoniates. 

 Confining ourselves to the green Algfe and the simpler Fungi we find among them 

 two sorts of phenomena which have been termed alternation of generations. Most 

 of these organisms reproduce both sexually and asexually, and sexual and asexual 

 individuals, resembling one another in their vegetative structure, are often found. 

 The same individual may, however, bear both kinds of reproductive organs, and 



• The existence of these recent statements of the problem renders reference 

 to the literature of the subject unnecessary. 



