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 rgo1 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 
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 1o 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- 
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 
*Fic 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, 7. e., deeper in this section, the bacteria fill 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. 
ing to the surface through the. 
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