196 



BACTERIAL DISEASES OF PLANTS 



toxins. You will find an abstract in "Bacteria in Relation to 

 Plant Diseases," Vol. Ill, p. 267. 



Write a description of the disease, drawn (of course) from 

 your own observations on either sunflower, tomato, potato, 

 or tobacco, or all four. If you have time, make a special study 

 of the dwarfing effects of the parasite. To what poison is this 



Fig. 138. — Cross-section of a potato stem in the vascular region showing 

 occupied by Bacterium solanacearum and the beginnings of a bacterial 

 cavity. Plant inoculated on a leaflet, May 27, 1895, and collected June 17. 

 .Stem sound externally. Block 113. X 800. 



due? Can you produce it with extracts of the bacteria: a by 

 injection, b by watering the soil? 



Histology. — Section, stain and study leaves and stems for 

 action of the organism on the tissues. Make good permanent 

 preparations. Are vessels occupied first? Are cavities formed 

 in the bundles? Have you found the organism in the phloem? 

 Study invasion of the pith, of the bark. Draw what you see. 



