BEADLE AND TATUM 



venient intervals. The frontier formed 

 by the advancing mycelia is remark- 

 ably well defined, and there is no dif- 

 ficulty in determining its position to 

 within a millimeter or less. Progression 

 along such tubes is strictly linear with 

 time and the rate is independent of 

 tube length (up to 1.5 meters). The 

 rate is not changed by reducing the 

 inside tube diameter to 9 mm., or by 

 sealing one or both ends. It therefore 

 appears that gas diffusion is in no way 

 limiting in such tubes. 



The results of growing the pyridox- 

 inless strain in horizontal tubes in 

 which the agar medium contained 

 varying amounts of Be are shown 

 graphically in figures 1 and 2. Rate of 



12 



10 



f 



o 



o 



o 



171 



progression is clearly^ a function of 

 vitamin B^ concentration in the me- 

 dium.^" It is likewise evident that there 

 is no significant difference in rate be- 

 tween the mutant supplied with Be 

 and the normal strain growing on a 

 medium without this vitamin. These 

 results are consistent with the assump- 

 tion that the primary physiological 

 difference between pyridoxinless and 

 normal strains is the inability of the 

 former to carry out the synthesis of 

 vitamin Be. There is certainly more 

 than one step in this synthesis and ac- 

 cordingly the gene differential in- 

 volved is presumably concerned with 

 only one specific step in the biosyn- 

 thesis of vitamin Be- 



0.002 0.004 0.008 0015 003 006 012 025 5 1.0 



Micro|rams Bj per 25 cc. 



Fig. 2. The relation between growth rate (cm./day) and vitamin Bg concentration. 



In order to ascertain the inheritance 

 of the pyridoxinless character, crosses 

 between normal and mutant strains 

 were made. The techniques for hy- 

 bridization and ascospore isolation 

 have been worked out and described 

 by Dodge, and by Lindegren.^ The 

 ascospores from 24 asci of the cross 



were isolated and their positions in the 

 asci recorded. For some unknown rea- 



!*> It is planned to investigate further the 

 possibility of using the growth of Neuro- 

 spora strains in the described tube as a basis 

 of vitamin assay, but it should be emphasized 

 that such additional investigation is essential 

 in order to determine the reproducibility 

 and reliability of the method. 



