6o 



BACTERIA IN RELATION TO PLANT DISEASES. 



that not every chance organism will produce this disease when introduced into the stomata, 

 but only Bad. pruni. 



In 1898 the writer pointed out that Bacterium stewarti probably enters the plant 

 through the water-pores situated at the tips of the leaves. The examination of diseased 

 maize-plants found in southwestern Michigan led to this assumption. The tips of many of 

 the leaves were dead, while the basal parts were living. The vessels in the tips of the leaves 



Fig. 13.* 



were occupied by the bacteria, which could be traced downward through the bundles 

 sometimes for a distance of one-third to one-half the length of the leaf and then disappeared. 

 This pointed clearly to apical infection. The rest was inference. Since that publication, 

 the writer has obtained numerous typical cases of the sweet-corn disease in Washington 

 (where the disease was not then known to occur naturally) by stomatic or water-pore 

 infections, one or both. Inasmuch as the sections thus far cut indicate infection by way of 



*Fio. 13. Bacterial black spot of the plum: 



Left: Pureculture inoculations. Plums from the sprayed tree of Abundance variety at Takoma Park, Maryland. 

 Photographed July 23, 1904, spots 53 days old. 



Right: Natural infections. Hale plums from the orchard at Duplain in Central Michigan. 



