276 MUTATION AND PLANT BREEDING 



reduction in fertility could be a great advantage. This already seems 

 to have been successfully attacked by Olmo (85). We have also had 

 our attention directed towards the same aim and have irradiated 

 plum varieties, like Victoria and Reine Claude d'Oullins, with the 

 intention of inducing some degree of sterility. It may be mentioned 

 in this connection that in many ornamental plants even complete 

 sterility, for several reasons, might be a highly desirable change, and 

 it should be a rather easy one to produce. 



Finally, I should like to touch upon two other projects that we 

 have also just started and therefore cannot say much about as yet. 

 The first concerns the induction of thornless sports in blackberries. 

 To that end we have irradiated seeds of an English variety, Merton 

 Thornless. which is said to be apomictic (53). Irradiation of apomicts 

 is a very interesting possibility, both because some of the induced 

 changes might then be constantly reproduced by seeds, but also 

 because the irradiation is known to be able to break down the 

 apomictic mechanism itself which might be desirable in order to 

 release the variation of apomictic species (52, 61, 62). 



In addition to mutagenic treatments with various radiations, we 

 have also performed experiments with injected radio-isotopes (33), 

 and also more recently with mutagenic chemicals. From the point of 

 view of mutation induction the treatments with P 32 were not very 

 successful, but this hardly permits us to say that this kind of treat- 

 ment would generally be so (111). 



The treatments with chemicals involved ethyleneoxide, ethylene- 

 imine, and ethylmethanesulphonate. Usually young trees, like those 

 shown in Figure 1, taken just when the buds begin to swell, have been 

 dipped in water solutions of the chemicals for 24 hours. 



Various chemicals that are active mutagenes in microorganisms 

 have turned out to be rather ineffective on higher plants as for exam- 

 ple the mustard gas substances (48). Chemical mutagenesis in these 

 plants has, therefore, until recently not been very promising. Ehren- 

 berg, et al. (35) found, however, that ethyleneimine (EI) in barley was 

 even more effective than X-rays and neutrons, 20 per cent compared 

 with 4 to 5 per cent mutations per spike progeny. EI did produce 

 some of the typical primary effects of radiations, e.g., yellowish 

 patches on the leaves of young apple seedlings, but not to the same 

 degree as X-rays. This year we made the treatments with ethylmeth- 



