170 



mate quantitative agreement with 

 those of table 1. It is clear that addi- 

 tional study of the details of culture 

 conditions is necessary before rate of 

 weight increase of this mutant can be 

 used as an accurate assay for vitamin 

 Be. 



It has been found that the progres- 

 sion of the frontier of myceUa of Neii- 

 rospora along a horizontal glass culture 

 tube half filled with an agar medium 

 provides a convenient method of in- 

 vestigating the quantitative effects of 



BEADLE AND TATUM 



growth factors. Tubes of about 13 

 mm. inside diameter and about 40 cm. 

 in length are used. Segments of about 

 5 cm. at the two ends are turned up 

 at an angle of about 45°. Agar medium 

 is poured in so as to fill the tube about 

 half full and is allowed to set with the 

 main segment of the tube in a horizon- 

 tal position. The turned up ends of the 

 tube are stoppered with cotton plugs. 

 Inoculations are made at one end of 

 the agar surface and the position of 

 the advancing front recorded at con- 



Fig. 1. Growth of normal (top two curves) and pyridoxinless (remaining curves) 

 strains of Neurospora sitophila in horizontal tubes. The scale on the ordinate is 

 shifted a fixed amount for each successive curve in the series. The figures at the 

 right of each curve indicate concentration of pyridoxine (B^) in micrograms per 

 25 cc. medium. 



