CYCLE NOT ALWAYS OBLIGATORY 65 



factors, external stimulus, light, temperature, moisture, access to oxygen, 

 and the chemical composition of the nutritive medium, have already been 

 recognised. These and others in various combination have been found, 

 or may in the future be found, to determine the succession of propagative 

 methods in many of the Thallophytes. 



But, on the other hand, accurate observation is showing in an increasing 

 number of examples that this freedom from obligatory succession of phases 

 is not universally the case in the Thallophytes. It is beginning to be 

 clear that here, as elsewhere, complications have arisen, associated with the 

 phenomenon of sex, which lead frequently to an obligatory succession of 

 phases, over which external conditions have little or no control. It has 

 been seen in the Archegoniatae and in the Phanerogams that the result 

 of sexual coalescence is a doubling of the number of chromosomes in the 

 subsequent nuclear divisions, with reduction as the final consequence. The 

 similarity in essentials of fertilisation in the Thallophytes to that in the 

 Archegoniatae is obvious : it has been found in many cases to result in a 

 doubling of the chromosomes in Thallophytes also, and this makes it seem 

 probable that there should be post-sexual nuclear complications of somewhat 

 the same nature in them also. Strasburger has drawn attention to the 

 impossibility of indefinitely continued doubling of chromosomes in fertilisation, 

 and the necessity for a reduction-process in plants which show sexuality : 

 we must assume that some process of reduction will sooner or later follow 

 in each life-cycle where sexual coalescence occurs ; but the mechanism of 

 the process, and the period at which it occurs in the life-cycle, may differ 

 in different cases. The differentiation of the sexes in the Thallophytes 

 has proceeded along many distinct lines. What is then more probable 

 than that in different lines of descent the problem of reduction, as a 

 necessary consequence of sexual fusion, should have been solved in different 

 ways, and at different points in the life-story? 



The facts observed in certain Thallophytes point to the conclusion 

 that this has actually happened : reduction has now been shown in some 

 of them to follow on sexuality, but its place in the life-cycle varies in 

 different cases. The point of interest for present consideration is not so 

 much the details of the process of reduction, as the place which it holds 

 in the life-cycles of various' Thallophytes, and the influence which it appears 

 to have had in determining in them an obligatory succession of phases. 



The question must for the present remain open how the reduction, 

 which we may presume to be a necessary consequence on fertilisation, 

 is carried out in those Thallophytes which show sexuality but have not 

 any fixed succession of phases, such as Vaucheria, etc. Subsequent observ- 

 ations will doubtless provide the actual facts, and will probably locate 

 the reduction-process either in near proximity to the germination of the 

 zygote, or it may be to the production of the gametes. 1 We may even 



1 Oltmanns, Morphol. it. Biol. d. Algen, 1904, p. 324; B. M. Davis, '' Oogenesis in 

 Vaucheria, " Bot. Gaz., 1904, vol. xxxviii., p. 81. 



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