nybom: vegetativelv propagated species 281 



of isolated mutations might be anticipated after irradiation at differ- 

 ent stages of development is shown by Heiken's results (55), but still 

 we know far too little for making general recommendations. The 

 method of decapitating rooted cuttings sometime before irradiation 

 seem to be worth attention (59, 94). As mentioned before, we have 

 been irradiating leaf-bud cuttings of blackberries rooted under mist. 

 It might be added here that the modern mist-propagation methods 

 seem to offer excellent possibilities for the management of irradiated 

 material of vegetativelv propagated plants, including the procedure 

 of repeated tipping for the isolation of the induced changes (93). 



The intrasomatic elimination is a process that should be coun- 

 teracted by all means. In barley we know that this loss of mutated 

 cells is more pronounced under some conditions than under other. 

 It is, for example, influenced by the hydration of the seeds, by tem- 

 perature, oxygen pressure, and dose, being more severe after high 

 doses (34). It plays a much smaller role after neutron irradiation 

 than after irradiation by X-rays or gamma rays (68, 83), which may be 

 one of the reasons why neutrons have been considered to be a more 

 effective mutagen than X-rays in fruit trees (10). In fact, this elimina- 

 tion is of great importance also for the yield of induced mutations in 

 seed-propagated plants. In peas, for example, it will tend to reduce 

 the part of the plant which is heterozygous for the induced mutation, 

 and according to Blixt and Gelin (15), this part is smaller, i.e., elimi- 

 nated to a larger extent after X-rays than after neutrons. The chemi- 

 cal mutagens tested during later years seem to follow the neutrons 

 more than the X-rays in this respect, which is, of course, much to 

 their credit (15). 



We have reasons to believe that this intrasomatic selection is 

 more pronounced in an object where there is great variation in radio- 

 sensitivity between the cells or between the primordia. The irradi- 

 ation should, therefore, be carried out at a stage when the tissue that 

 afterwards is to give rise to the new plant consists of as few and 

 undifferentiated cells as possible. 



By introducing chronic irradiation we originally thought we 

 might be able to differentiate between mutation induction, showing 

 no dose-rate dependence, and the probable physiological and dose- 

 rate dependent damage leading to intrasomatic elimination. Thereby, 

 we should get an accumulation of mutations above the maximal 



