HYBRID NUTRITIONAL REQUIREMENTS 



119 



These results suggest that there is present in the grains of corn, material 

 which stimulates the early growth of Phycomyces in the presence of thiamine, 

 and that there is more of this material per grain in heterotic hybrids than in 

 those of the inbred parents. 



Interpretation of these results depends in part on the identity of factor Z. 



100 



90 



80 



O 



D 

 _1 



u 



O 



>- 



H 



>- 

 tr 

 a 



70 



60 



50 



40 



30 



20 



10 



025 



0.5 

 ML. CORN GRAIN EXTRACT 



Fig. 7.2 — Increase in dry weight of Phycomyces produced by extracts of air dry grains of 



maize. Extracts added to medium of sugar, minerals, asparagine, and thiamine. A = line 



4-8; B = line 187; C = 985, 4-8 X 187; D = 995, 187 X 4-8. 1 ml. extract = 1 grain. 



Unfortunately, we do not know what factor Z is. We succeeded in dividing it. 

 We demonstrated that factor Z is multiple, and separated it into a fraction 

 adsorbed on charcoal, factor Zi, and a filtrate fraction, factor Z2. Factor Zi 

 was identified as hypoxanthine. Factor Z2 may be a mixture of amino acids. 

 Although this problem is left in an uncertain and unsatisfactory condi- 

 tion, it suggests a line of attack. This would be an investigation of heterosis 

 by studying the efifect of extracts of parents and of heterotic hybrids on the 

 growth of other organisms. This may serve as a means of bioassay for favor- 

 able or unfavorable growth factors. 



