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^ xj'24 THE LIFE-HISTORY OF A FERN [CH. 



nuclei of the spores {k), are seen to be haploid, and they start the game- 

 tophyte generation with nuclei containing 64 chromosomes. This is then 

 continued in the gametophyte, till syngamy again doubles the number to 

 128, and establishes again the diploid sporophyte. 



The alternating nuclear cycle thus demonstrated coincides in normal 

 cases with the alternating phases of sporophyte and gametophyte, as defined 

 by external form. The demonstration of this for Ferns, and the fact that it 

 is so for Archegoniate Plants at large, greatly intensifies the interest which 

 naturally attaches to the phenomena of alternation. Rut it cannot be assumed, 

 from the fact that the chromosome-alternation applies accurately for normal 

 cycles, that that succession of events will always be maintained. An ever 

 increasing number of individual cases has been recorded in which the events 

 of syngamy and of spore-production, with which the doubling and halving 

 of the number of chromosomes are involved, may be excluded from the life- 

 history. Such conditions are described respectively as apomixis (or apogamy), 

 and apospory. It is naturally to be expected that in such cases the chromo- 

 some-cycle will be disturbed, and cytological investigation shows that it is 

 so. They will be considered in detail in Chapter XVI. Meanwhile it may be 

 held that the prevalence of the cycle of events as here described, whether 

 as regards the form of the alternating generations or the chromosome- 

 numbers, is the normal. These departures from it may be held as late and 

 sporadic incidents. The natural inference will therefore be that the normal 

 cycle, as described in this Chapter, was that which held good throughout the 

 evolution of the Filicales. In essential features it corresponds to a similar 

 cycle demonstrated in certain Brown and Red Algae. The materials are 

 therefore present which might stimulate theoretical discussion of the evo- 

 lutionary origin of an alternation of this nature, and of the possible priority 

 in Descent of the one generation or of the other. But the time is not ripe 

 for any decision of such questions : for it is quite possible that the alternations 

 seen in the several distinct phyla of Algae, and that seen in Archegoniate 

 Plants, may really be homoplastic in all these cases, and not truly homo- 

 genetic at all. Much more detailed comparative study will be necessary 

 before any assured opinion on such points can be evolved. 



The normal alternation appear^s clear-cut in the Archegoniatae at large, 

 and evidences of nascent stages of it are deficient in them. Consequently 

 the Archegoniatae are not likely to provide the necessary early steps which 

 would illuminate the progression towards that alternation which is seen in 

 them. The most promising field would appear to be the methodical study 

 of the cytology of those phyla of Algae in which the most elaborate forms 

 show a well-marked alternation, while in the simpler types the alternation 

 is either rudimentary, or even absent. But this book deals with the Filicales, 

 not with Algae: and the question must therefore be left over for the Algo- 



