THE BEAN BLIGHT: TECHNIC 



289 



stomata. Indeed, it is one of the most convenient diseases for 

 studying stomatal infections. 



The needle-pricks should be made on young pods, soft stems, 

 and various parts of the immature leaf (petiole, petiolule, veins 

 and parenchyma) . For comparison try also needle inoculations 

 in full-grown leaves, stems and fruits. 



For the spraying experiments, plants a foot high, bearing 

 mature, immature and undeveloped leaves, should be selected, 



FIG. 223. Bacterial cavity in a bean leaf 5 days after spraying on a pure 

 culture of Bacterium phaseoli. Tissue not yet collapsed. Stomatal infection 

 through S, S. 



so as to observe the modifying influence on the disease of age of 

 tissues. The leaves should be free from insects, fungi, and con- 

 fusing spots of any sort. The plants should be atomized with 

 water holding in suspension bacteria from young (3-day) agar- 

 streak cultures until the surface is covered with small drops of 

 the cloudy fluid. Then the cages should be closed tightly and 

 protected from the light. They should be examined every few 

 hours throughout the daytime to see that the drops have not 

 evaporated. To insure this favorable persistence of moisture 



19 



