BLACK ROT OF POTATO: HISTOLOGY 277 



occupy them incidentally to the general destruction of the paren- 

 chyma? Compare with sections of potato plants attacked by 

 Bacterium solanacearum. Study the disintegration of the tuber. 

 Are cavities formed? What is the earliest stage of the rot? 

 What becomes of the starch? Of the cell-wall? Is any gas 

 formed out of the tissues of the potato? Compare with Bacillus 

 carotovorus (No. VI). Inoculated, rotting tubers placed at 5°C. 

 for 10 days offer a good opportunity to study the reduction of 

 bacterial growth and the development of a protective cork 

 barrier (See also No. VI). Cut sections until you have made 

 out the newly formed cork layer clearly. Use some good cork 

 stain if necessary. Make sections through lenticels on the tuber 

 in very early stages of infection to show the bacterial penetra- 

 tion. Cut on the microtome from paraffin-infiltrated material. 

 These sections should be made not much later than the third 

 day, i.e., as soon as a trace of infection is visible on the surface 

 (using a hand lens). A few days later the lenticel infection is 

 much more conspicuous, but then the bacteria have generally 

 passed several millimeters beyond the region of the lenticel. 

 Determine if you can how the bacteria enter the stem, i.e., are 

 the stem infections stomatal, or only by way of wounds? Spray 

 and look for stomatal leaf infections. Appel left this matter 

 undetermined. I failed in two attempts. 



Variability. — All varieties of potatoes are said to be subject 

 to this disease. Have you been able to find differences, either 

 from your inoculations, or from field observations? Consider- 

 able time devoted to such an inquiry might be well spent. It 

 certainly would be if practical results were forthcoming, or even 

 suggestions toward such results. The Early Rose, Imperator, 

 Maercker, Magnum Bonum, and Wohltmann (sorts commonly 

 grown in Germany) are frequently attacked (Appel). The 

 writer found Daisy, Green Mountain, and Factor quite suscep- 

 cible (see "Bacteria in Relation to Plant JDisease," Vol. II, 

 Plate 1 1 — facing p. 96) . White McCormick is also quite suscep- 

 tible (Figs. 195 and 211). Are potatoes the fleshy part of which 



B. Cross-section of stem of Tropaeolum majus (nasturtium) attacked b}' 

 Bacterium solanacearum, showing brown stain and bacterial ooze. Plant from. 

 Baltimore, Md. Photographed July 22, 1914. X 8 circa. 



