198 



BACTERIAL DISEASES OF PLANTS 



Does the organism come freely to the surface of the attacked 

 plants (Contrast with No. V or XII) ? Does it freely attack the 

 root-anlage (Contrast again with No. V)? Stomatal infection 

 is easily obtained and studied on leaves of the Tropaeolum. 

 Check Tropaeolums in the same pot are frequently attacked 

 after some weeks. Consult Figs. 136 to 142. 



Variahility. — How long does an attacked plant live? Have 

 you seen indications of immunity on the part of inoculated 

 plants? of recovery? Do you think rapid growth or great 

 juiciness favors the progress of the disease? What results 



.''frjrr 



Fig. 140. — A detail from same series as Figs. 138, 139, showing the individual 

 bacteria. X 1000 circa. 



did you obtain from your trials on young vs. old plants? Can 

 you increase susceptibility by overwatering or decrease it by 

 liming the soil? or by the liberal use of potash and phosphates? 

 What do you conclude with reference to the effects of tempera- 

 ture? Does the optimum temperature for the plant coincide 

 with that of the micro-organism? Have you found any non-sus- 

 ceptible varieties? A good non-susceptible tobacco would just 

 now be worth its weight in gold! Why is the disease common in 

 our Southern States and unknown or hard to find in our Northern 

 States? Can you determine its presence in states north of 



