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THE AMERICAN NATUBALIST [VoL. LV 



It is possible that a siiigie set in an otherwise tetra- 

 ploid plant may have an extra chromosome, giving 5 

 chromosomes in one set and 4 in the remaining eleven. 

 At least we have a single plant in a tetraploid pedigree 

 which strongly resembles the Grlobe— the besit known of 

 our simple trisomic mutants. The cytological evidence 

 shows that its chromosomal number is at least tetraploid, 

 but is not yet sufficient to prove that its Grlobe-like ap- 

 pearance is determined by the addition of a fifth member 

 to the chromosomal set responsible for the Globe mutant. 



The occurrence of mutations of the types discussed in 

 the foregoing paragraphs is bound up with the causes of 

 chromosomal duplication. Knowing the mechanism to be 

 affected, we may be able ultimately to induce chromo- 

 somal mutations by the application of appropriate 

 stimuli. 



We have outlined the types of chromosomal duplica- 

 tion already found in the jimson weed, and have shomi 

 some of the ])ecnlinrities in the ])reeding behavior of the 

 mutant forms which tlicv condit ion. It will be well to 

 consider I'oi- .-i iiioiiirnt this process ot' (hii)licntion as it 

 affects the iii(li\ i(hial phuit and as it nmy liavc a possible 

 significance in our theories of mutation and cxoiution. 



The mutants of the Poinsettia or (ilolif i \ pc. in which 

 but a single chromosomal set is involved in the duplica- 

 tion, should enable one to discover something in regard 



