104 Discussion on “ Alternation of Generations. 
DISCUSSION ON “ALTERNATION OF GENERATIONS” 
AT THE LINNEAN SOCIETY. 
1 1HE Meeting of the Linnean Society of London on February 
18th, 1909, was devoted to a discussion on this subject, 
following a paper by Dr. W. H. Lang. A general desire was 
expressed that the paper and discussion should be reported in extenso 
in the New Phytologist. The paper and most of the speeches 
which followed are reported practically verbatim as they were 
delivered, but Professor Bower and Professor Farmer preferred to 
supply their own versions. The Editor is much indebted to Miss 
Agnes Robertson, who kindly took careful and very accurate notes of 
the speeches. The reports based on these notes have been revised 
by the speakers for publication. 
Dr. Lang spoke as follows :— 
» 
I should like in the first place to acknowledge in a word my 
sense of the honour of being invited to contribute this opening 
paper to the discussion to-night, and, as time is limited, shall address 
myself at once to my subject. 
We are all clear as to what we understand by the well marked 
alternation of generations in the life-history of a Fern or a Liverwort. 
I shall confine my remarks to points bearing on the origin of 
alternation of this type, and not enter into the wider general question 
of the origin of alternation of generations at large. Time will not 
permit of summarising the diverse views held on the subject, but 
these will be better dealt with by others in the course of the 
discussion. 
Not only are the sexual and spore-bearing generations clearly 
different in form in archegoniate plants, but there is a constant 
cytological difference between them. The cells of the spore-bearing 
or diploid generation have nuclei with twice the number of chromo¬ 
somes of those forming the sexual or haploid generation. In certain 
Algae such as Dictyota we find a corresponding alternation of sexual 
and spore-bearing individuals with the same cytological difference. 
Alternation of generations presents us with a problem in 
ontogeny as well as in phylogeny, and it is possible that the phylo¬ 
genetic problem may be best attacked through the ontogenetic. I 
shall ask you to consider briefly how we are to look on the ontogeny 
of an organism with two alternating generations in its life-cycle ; 
this appears to present difficulties that have hitherto not been fully 
faced. It is more complicated than the ontogeny of an organism 
without alternation. In this we can look on the germ-cell giving 
rise to the individual as a specific cell, the nature of which, under 
the usual conditions, determines the result. Each cell of the 
individual would correspond to the specific cell and any cell of the 
body would therefore be theoretically capable of reproducing the 
organism. We may regard the production of the individual from 
the germ-cell as due partly to the external conditions and partly to 
internal correlations and the mutual influence of the tissues. 
In the life-cycle of a plant with definite alternation of 
generations we meet twice with a germ-cell capable of giving rise 
