which might be expected to reduce metabolic activity also reduce sen- 

 sitivity. These include reduced oxygen tension, low temperature, and 

 nonlethal doses of respiratory poisons. There are, however, exceptions in 

 which these normally "protecting" conditions actually increase sensi- 

 tivity. A number of chemicals are known to increase or decrease peroxide 

 formation, the former generally enhancing and the latter generally de- 

 creasing radiation effects. However, no general valid principle can be 

 set forth at the present time. 



(a) 



Figure 9-3. (a) Tetraploid Microspore Metaphase with a High Degree 

 of Fragmentation Resulting from Irradiation with 100 r of X Rays at First 

 Meiotic Metaphase. (b) Drawing of Tetraploid Metaphase Illustrated in (a). 

 The chromosome drawn in black appears to be monocentric and is about 

 four times the length of the longest chromosome in a normal complement. 

 It could include a triplication. The two arrows indicate the approximate 



(Contin. on opposite page.) 



198 / CHAPTER 9 



