92 



BACTERIA IN RELATION TO PLANT DISEASES. 



farm without due consideration of what may happen. Certain bacterial diseases 

 might be distributed very readil)' in this way and good fields rendered worthless 

 for certain crops. 



The parasite may gain entrance to the plant through wounds (plates 2 and 4 

 and fig. 8) or by way of the stomata (figs. 70 to 75), lenticels, water-pores (figs. 76 

 to 79), and nectaries. In recent years the writer has discovered a number of very 

 characteristic infections by way of the stomata and the water-pores, which are only 

 modified stomata, e. g., in cabbage, mustard, plum, bean, soy-bean, cotton (fig. 80), 



Fig. 76.* 



pelargonium, larkspur, broomconi, sorghum, maize, cucumber, etc. Pear-blight 

 affords one of the most striking examples of wholesale infection b}^ wa}' of the nec- 

 taries. The wilt of cucurbits affords an equally good example of infection through 

 wounds — namely, leaf-injuries due to beetles. 



*FiG. 76. — Baclcriiim raiiiffcslrc. Section of a cilibage leaf parallel to the surface and near the 

 margin, showing the result of infection through the water-pores. The tissues are browned and de- 

 stroyed. Immediately under the Icaf-serrature a cavity has formed and the bacteria have begun to 

 penetrate into deeper parts of the leaf by way of the spiral vessels, not all of which are occupied. 

 This figure is slightly diagrammatic, but only to the extent of omitting the protoplasmic contents 

 of the parenchyma cells and of introducing six occupied spiral vessels which belong to the next 

 section in the series. No spiral vessels are visible in the lower part of the section because the 

 knife passed just below them. Material collected on Lf>ng Lsland, July 16, 1902, and (Ixed in strong 

 alcohol. The spirals here shown are a lillle too densely occupied by the bacteria to make a good 

 drawing under the oil-immersion objective, but a little f.irtber in (beyond Xjthcy are less abundant 

 and entirely satisfactory for this purpose. 



