TRANSACTIONS OF SECTION K. 1031 



organ formed at a comparative!}' late period ; and it corresponds in position and 

 function, thoug-h not in time of origin or details of structure, with that of Selagin- 

 Ma. I conclude that the ' foot ' of Selaf/ineUa is probably a later formation, not 

 comparable as regards descent either -with the foot of Lycopodium or with the 

 * feeder ' of JVehcitschia ov Gnetum. The latter are plainly of recent independent 

 origin as comparison shows, and their actual position is defined according to the 

 position of the seed in germination. Probably, then, there is homoplasy in such 

 cases, not true homogeny. 



Similarly with such structures as the pinnae, stipules, iudusium, corona, and 

 still more so with such inconstant bodies as emergences and hairs ; when we speak 

 of the ' homologies ' of these parts it is rarely the homogeny, or identity by descent, 

 which we mean to express ; usually it is only homoplasy, a comparison of parts 

 similar it may be in form and position, or even in development and function, 

 though not shown to be comparable by descent. 



Alternation. 



But the questions above discussed are mere matters of detail, compared with 

 that great enigma of the alternation of generations in green plants, or of alterna- 

 tion at large. This is, after all, a question of degree of homology, not now of the 

 parts only, but of the whole plant or ' generation.' How this greatest of all adapta- 

 tions was really initiated, we cannot expect to bring to the point of demonstra- 

 tion; at best we can only venture opinions of probability. Still, this discussion 

 commands at present more widespread interest among botanists than any other in 

 the sphere of plant morphology. 



There was a time when the attempt was made to reduce all plants to one 

 scheme as regards their life-cycle, a method which not only prevented elasticity of 

 theory, but was responsible for some unfortunate comparisons. It was character- 

 istic of the period when the text-book of Sachs reigned supreme ; we find it there 

 definitely laid down that ' the doctrine of alternation has the object of reducing to 

 one scheme the main phases of the life of all plants which bear sexual organs.' 

 But the controversy between Pringsheim and Celakovsky had, as one of its 

 results, the recognition of various types of life-history, not of one scheme only. 

 The tendency at present is towards the opposite extreme ; the frequency of the 

 parallel developments now recognised has led some to accept a comprehensive 

 polyphyletic view as regards alternation, and, wherever difficulties of comparison 

 arise, to take refuge in the plausible suggestion that the organisms compared 

 represent altogether distinct lines of descent. But the view which should be 

 confidently upheld is that even where this may actually be the case useful com- 

 parisons may yet be made ; and that the method of progress within one phylum 

 may illustrate the probable mode of progress in another. The green Algte may 

 thus throw light upon the probable origin of the sporogonium in the Bryophytes, 

 though they may in no sense be in tlie line of their descent ; the Bryophytes may 

 suggest valuable ideas for the comparative study of the Pteridophytes, though 

 they may not represent their actual ancestry. 



It is the alternation as seen in these green plants that I propose to discuss. 

 Writers have distinguished various types of alternation, including under the term 

 •divers modes of 'alternation of shoots;' and it should be remembered that this 

 was the original sense of the word alternation as applied l)y Steenstrup. But 

 gradually the issue in the case of green plants has been simplified, and the ques- 

 tion now centres round that alternation of phases which some of us describe as 

 ' antithetic,' while others believe the phases to be really ' homologous' as regards 

 their origin. 



Briefly put, the question is. How was the first start made ? Has the neutral 

 generation or sporophyte been the result of change of any other part of the sexual 

 generation than the zygote itself .^ If so, the alternation is of homologous genera- 

 tions ; if not, then the alternation is what is styled cuitithetic. The whole discus- 

 sion is like a purely historical inquiry, but with the minimum of documentary 

 evidence ; for on this point the fossils give scanty help. In the absence of mora 



