HEREDITY AND MUTATION AS CELL PHENOMENA 523 
closely parallel mutations in which the chromosome series is doubled — 
28 — the plant being a cell giant and not merely gigantic in its external 
dimensions. This mutation has not only occurred independently in 
two different strains of Oe. Lamarckiana, but a somewhat different 
giant having the same chromosome number occurred in Heribert- 
Nilsson's cultures of the Swedish Lamarckiana. Recently Bartlett^ 
has found a tetraploid giant in his experiments with Oe. stenomeres 
which is an exact parallel to the previous cases in every particular. 
A third series of morphological mutants is the semigigas series, 
having 21 chromosomes. This type of mutation has now been found 
in several species, including Oe. Lamarckiana, Oe. biennis, and hybrids 
of Oe. Lamarckiana or its derivatives with such species as Oe. cruciata, 
Oe. muricata and Oe. Miller si. Triploidy is thus much more frequent 
in its occurrence than tetraploidy, though the latter condition was 
discovered first. 
Derivative from the above three types of chromosome change are 
various others. Thus a mutant from Sweden which I have called 
latescens^ probably has 16 chromosomes, and by crossing and other- 
wise several additional numbers have been obtained. 
The truth is obvious, not only that parallel mutations occur inde- 
pendently in different species, but that the type of change which 
gives rise to the extra-chromosome series is entirely different from 
that which produces the tetraploid series. I wish to point out that 
the nature of these changes is probably limited, and in this sense 
determined, by the structure of the germ plasm. In one group of 
organisms the mutations are of one kind, in another group they are 
wholly different in nature. This must be because the germ plasm in 
each group has its own particular lines of cleavage. Tetraploidy is a 
phenomenon which has occurred in a great variety of organisms, 
because the nuclear structure of almost any organism allows of its 
occurrence. Chromosome duplication, as in the lata series, is appar- 
ently much less common, but it may be expected to occur wherever the 
pairing of chromosomes in meiosis is weak and therefore liable to 
irregularities. Mutations then, in a sense, indicate where lines of 
weakness exist in the germ plasm, and it is these lines of weakness 
which define the particular directions which the mutations will take 
* Bartlett, H. H. 1915. The mutations of Oenothera stenomeres. Amer. 
Journ. Bot. 2: 100-109, figs. 4. 
^ Op. cit., p. 117. 
