78 MUTATION AND PLANT BREEDING 



long unspiralled arms behaving like bridges. Either of these effects 

 would be more likely to occur when stickiness is also present. 



d. Persistent nucleoli. — These sometimes occur in irradiated 

 cells and since nucleoli are known to modify chromosome mechanics 

 (40, 101), it seems quite likely that cells with persistent nucleoli 

 might behave differently with respect to terminal ization of chiasmata 

 and separation of chromosomes at anaphase. Irradiation of nucleoli 

 by microbeams of ultraviolet for periods as short as 3 seconds can 

 cause permanent stopping of cell division in Chortophaga neuroblasts 



(48). 



e. Nuclear enlargement (without polyploidy). — This change 

 sometimes occurs in irradiated cells and may have cytogenetic conse- 

 quences if additional exposure occurs. (See Sparrow and Forro (164) 

 for references.) 



f. Biocliemical, cytochemical, or histo chemical changes. — Many 

 changes falling in this category are known. Some may be the result 

 of primary damage to the genetic system but others almost certainly 

 are not. Many examples are known (7, 38, 53, 1 18, 125). 



V. Effects on Growth Rate, Growth Habit, Reproductive 

 Capacity, and/or Phenotype 



1. Growth rate 



Growth rate may be retarded, completely stopped, or, in some 

 cases, stimulated. Reduced growth, one of the commonest effects seen 

 in plants, increases with increasing doses. However, different species 

 differ greatly in the dose required to produce a given effect. The exact 

 cause of the growth inhibition is difficult to pin down since the 

 amount of inhibition can be correlated with both cytogenetic damage 

 (18, 125, 163) and with a drop in level of auxin (53). In some cases 

 buds may be inhibited for very long periods (161) without showing 

 visible breakdown. Accelerated growth, growth stimulation, or pre- 

 cocious maturation have also been reported in many plants (14, 62, 

 125, 160). The nature of these growth-enhancing effects are not 

 understood, but probably result from physiological disturbance rather 

 than direct cytogenetic alterations. 



2. Abnormal growth habit including tumor induction 



This category is distinguished from the previous one by an 

 abnormal growth pattern or growth habit rather than inhibition, and 



