58 



BACTERIA IN RELATION TO PLANT DISEASES. 



(notices in Science). This disease offers an excellent example of a natural infection, in 

 which insects and other wound-makers play no part except possibly as common carriers, of 

 which there is as yet no evidence. I did, indeed, suspect when the first specimens were 

 sent to me in 1901 that this disease might be spread by the punctures of insects. This was 

 owing to certain little cracks discovered in the center of some of the spots. There was no 

 distinct evidence, however, of insect injuries except certain curculio stings, which had 

 healed over and were sound. Later, when I visited the orchard and had excellent oppor- 

 tunities for studying the disease on thousands of plums, this hypothesis had to be 

 abandoned as wholly untenable, the little cracks being found to be due to entirely different 

 causes. Moreover, numerous serial sections which have been made in my laboratory show 

 clearly that the disease does not begin in wounds. The earliest stage is simply a bacterial 



occupation of the substomatic 



w 



chamber (fig. 12). A little later 

 a few neighboring cells are in- 

 volved, and then we have the 

 condition shown in vol. I, fig. 70. 

 This stage precedes the appear- 

 ance of spots, but with the grad- 

 ual multiplication of the bacteria 

 deeper tissues are involved, a 

 small closed cavity filled with 

 bacteria is formed, and the dis- 

 ease manifests itself externally 

 by a minute water-soaked spot 

 surrounding a single stoma and 

 best seen by the use of a hand 

 lens magnifying 8 or 10 times 

 (plate 3, fig. 1). In this stage 

 the epidermis is uninjured and on 

 the fruit the spot bulges slightly 

 from pressure of the growing bac- 

 terial mass underneath. A little 

 later we have the bacteria escap- 

 ing to the surface through the 

 stoma or through a slight central 

 rupture as shown in plate 4 and 

 vol. I, fig. 72. Gradually the bac- 

 teria burrow deeper and especially 

 wider, the spot enlarges, becomes 

 black and sunken, and the bac- 

 teria find their way to the surface through many stomata (plate 3, figs. 5, 6, 7), as well as 

 through the gradually enlarging central rift. The writer has numerous stained sections 

 showing all stages in the progress of this disease. This material was fixed in strong alcohol, 

 infiltrated with paraffin, and cut on the microtome. The cracks observed originate from 

 the drying out of the spots, from pressure exerted by the bacteria multiplying in the deeper 

 tissues, and, in later stages, from tensions set up in the dead tissues by the rapidly 



F.g 12.* 



*Fig 12. Black spot on green Japanese plum: Earliest stage of infection by Bacterium pruni; bacteria confined 

 to sub-stomatic chamber A pure-culture infection, twelfth day; from the Takoma Park tree. Slide 308 C16, 2d 

 section from right, middle row. Drawn with a Zeiss 3 mm. apochromatic 1.40 n. a. objective, No. 12 ocular and Abbe 

 camera. 



At a focus a little lower down, i. e., deeper in this section, the bacteria till the whole sub-stomatic chamber. In 

 sections to either side of this one the bacteria are limited, as here shown. Extremely fine dots within cells represent 

 protoplasmic masses and not bacteria ; the rounded and spindle-form, large, dark masses are nuclei. 



