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 lifecycle 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 
neces usion, should hi zed in different 
ways, and at different—pointsiathe life-story ? 
The facts observed in certain Thallophytes point to the conclusion 
that this has actually happened: reduction—has—now—been shown in some 
em : arty its~place.in the life-cycle varies in 
Miiarsat caseer-"Thie polit of iniesest tr present consideraiion I nor sc" SO 
“auch the details of the process of reduction, as—theplace which it holds. 
the li es,_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.! We may even 
1QOltmanns, Morphol. u. Biol. d. Algen, 1904, p. 324; B. M. Davis, ‘* Oogenesis in 
Vaucheria, ” Bot. Gaz., 1904, vol. xxxviii., p. 81. 
E 
