488 MUTATION AND PLANT BREEDING 



the microorganisms for chemical interpretations even though recom- 

 bination data to date are intelligently interpretable mainly by analogy 

 with behavior demonstrated by higher organisms. 



At the chromosome level, the gap between the lower and higher 

 forms needs to be bridged as Doctor Smith has pointed out, and 

 he has suggested fungi as bridging organisms. Other problems left 

 to the future are the relation of the mechanics of replication of the 

 cistron to the duplication of the chromosome and its relation to 

 recombination. Although a great deal is known about recombin- 

 ation, and the intrinsic and extrinsic factors which modify it, the 

 main physico-chemical key is still lacking. 



At the nuclear level the relation of cistron structure to the 

 structure of the presumed related enzymes through which effects 

 are mediated needs to be worked out. 



At the cellular level, the inter-relations between gene and cyto- 

 plasm need clarification. In this important area, practical plant 

 breeding utilization of the phenomenon of cytoplasmic male ste- 

 rility has far outstripped the explanation of its cause. 



At the organism level, the problem of differentiation of tissue 

 is likely to be intriguing for sometime to come. At present it is 

 difficult to define even the nature of the problem. Is it one of pro- 

 gressive gene mutations precisely controlled by special gene-regu- 

 lating genes? What triggers the reaction of morphogenic factors? 

 Do the morphological mutants represent gaps in the normal gene 

 chain of command, or are they normal genes reacting normally but 

 out of phase? Because they are so common, the chlorophyll mutants 

 have tended to be disregarded except as tools to measure mutation 

 induction efficiency and specificity. Discovery of the cause and the 

 correction of some of their impaired functions might provide poAver- 

 ful tools for controlling physiological states within the plant and its 

 concomitant effects on development. 



The organism and population levels are of most concern to 

 plant breeders, to ecologists, to taxonomists, to evolutionists, and 

 to population geneticists. The primary concerns are (a) the rela- 

 tionship of genotype and response to specified environments, and 

 (b) the distribution of genotypes among plant populations. Inter- 

 locus specificity with respect to genetic contribution to form and 

 performance has been amply confirmed since Mendel's treatment 



