THE BEAN BLIGHT: HISTOLOGY 295 



cut better if the air is pumped out of them when they are first 

 put into the acid-alcohol or other fixative. 



Do the same things with the leaf spots. Is there any 

 increase in the number of chloroplasts in cells surrounding the 

 leaf spots ? To what is the russet or rusty-red phenomenon due ? 

 What becomes of the cell-walls? How is the middle lamella dis- 

 posed of? Ziehl's carbol fuchsin is a good stain. 



Is the tissue killed in advance of bacterial occupation? 

 What are your reasons for thinking so? 



Fig. 228. — Agar-poured plates of Bacterium phaseoli (Idaho isolation). Inner 

 one-half of each exposed to bright sunlight on ice: a, 2 minutes; b, 5 minutes, and 

 then incubated for 6 days. Much less sensitive than Bacterium malvacearum. 

 Compare with Fig. 249. Photographed June 16, I9I5. y^ nat. size. 



Does the organism frequently penetrate the seed coats? 



On stem-inoculated plants how far can you trace the bacteria 

 in the vascular bundles? Is the phloem attacked? 



Variability. — Under field conditions different varieties of 

 beans show marked differences in susceptibility. If the student 

 has opportunity he should study variability in the field and 

 garden, being always on the lookout for resistant varieties 

 and hardy individuals in susceptible varieties. Make careful 

 (and legible) pen notes of what you have seen. 



Transmission. — We know nothing concerning living carriers 

 of the organism but, owing to the free oozing of the bacteria 

 to the surface, any bird or insect might act as a carrier from 

 diseased to healthy surfaces, as in case of fire-blight of apple 



