THE MULBERRY BLIGHT: SIGNS 357 



spots to the blackening of leaves and shoots. How far down 

 the stems in advance of external signs can you trace an internal 

 stain? In what tissues? Effect of the bacteria on the tree 

 as a whole. 



Describe the disease. Make photographs or drawings. 



Histology. — Cut, freehand, various cross-sections and longi- 

 tudinal sections of affected stems, and examine at once in water. 

 Fix, embed, section and stain suitable pieces of stem and leaf 

 in early stages of the disease to show the bacterial invasion. 

 Make permanent preparations. Study ooze of the bacteria 

 through lenticels; section young stems some inches below 

 external signs of the blight for presence of tyloses in the affected 



Fig. 275. — Agar surface colony of Bacterium mori showing internal markings 

 when viewed by transmitted oblique light. Colony smooth on the surface. 

 X 23. 



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vessels. Can you find any in normal shoots of this age? Com- 

 pare with tomato and potato stems attacked by No. IV or VI, 

 which also show tyloses. What substances cause their produc- 

 tion (see Figs. 357, 358 and 359 for tyloses induced by purely 

 chemical means) ? How many inches can you trace the bacteria 

 downward in the vascular system below the lowermost external 

 indications of the disease? Can the tyloses be traced farther 

 than the bacteria? What stem- tissues are specially involved? 

 For answer to this question examine both the soft terminal part 



