XIV. X RAYS AND X 1 H 11 A D I A T 1 O N 469 



corn has the disadvantage of a longer generation time and a less 

 easily handled C3''tological technique. Materials like the pollen 

 grains of Tradescanha (4-8,50) have proved particularly useful in the 

 stud}'' of chromosomal rearrangements due to radiation. These pol- 

 len grains have the advantages of being good cytological material; 

 the cytological patterns of cell division, as in mitosis and meiosis, are 

 well understood and standard. The disadvantages appear when the 

 results require progeny studies. 



Other materials, like bacteria {24,25,27), have the advantages of 

 being single cells, of discreteness, and of high rate of reproduction. 

 They are quite useful for studying the effect of radiation on survival, 

 provided the species is such that the cells do not form chains or clumps. 

 They may be used for the study of problems of frequency of pheno- 

 typic mutations, but they have the difficulty that very little is known 

 about their method of chromosome reproduction, with the conse- 

 quence that interpretation of the results on a comparable basis with 

 those in other forms, in which cytological behavior is known, may 

 lead to false conclusions. Viruses, as the plant viruses of the tobacco 

 mosaic group {23), are very useful for X-ray study in that, like bac- 

 teria, they may be plated out as individuals and the changes observed 

 in them noted and carried on as genetically different strains. They 

 are useful for the study of the lethal effects of different types of radi- 

 ant energy and for determining size of entities in the organism and 

 comparing the sizes to those obtained through other methods. The 

 \'iruses, however, have the same difficulties as bacteria, in that their 

 mode of reproduction is not understood sufficiently to interpret 

 many of the results. In the study of nuitations, both natural and 

 due to radiant energy, the bacteriophage types have been useful {16). 

 They have given real light on the mechanism of mutation through the 

 changes of successively mutated strains. Again, the difficulty is 

 lack of knowledge of the mechanism involved in the transmission of 

 their inheritance from one indixidual generation to the next. In the 

 study of mutation frecjuencies and the types of mutations produced 

 by radiant energ\', ihc different fungus forms have proved useful. 

 This is particularl}' true of the bread mold Neurospora {7,37), and of 

 yeast {38), in which the cytological arrangements of the reduction 

 division are such as to return all of the products of the maturation 

 divisions for evaluation of the inheritance they receive. 



Radiation studies on multicellular organisms have been useful 

 for examination of particular physiological problems, for instance. 



