64 BACTERIA IN RELATION TO PLANT DISEASES. 
passage-ways into the deeper tissues of the plant, and it is probable that bacteria frequently 
make use of them in canker-diseases and the like, but of this we have, as yet, no clear 
proof. Incase of potato-tubers, the lenticels swell and rupture when the earth is unduly 
moist or when they are kept in a saturated atmosphere, and an easy entrance is then afforded 
to all sorts of soil organisms, es- 
pecially to certain bacteria which 
induce soft rots. The writer has 
occasionally seen on the potato- 
tuber a small superficial bacterial 
rot-spot centered in a single lenticel. 
Sorauer observed this lenticellate 
infection many years ago. It was 
also noted by Reinke and Berthold 
in 1879, and has been seen by other 
persons. The writer first observed 
it in the laboratory in 1886-87, 
during a winter spent on diseases of 
the potato, and has seen it in the 
field in wet autumns. The earliest 
record of any sort of lenticellate 
infection appears to be that of 
Hermann Schacht. In 1856 he 
stated that scab often begins in the 
lenticels of the potato-tuber (Joc. 
cit., pp. 24 to 25, and his plate VII, 
fig. 3), and early the following year 
Caspary also called attention to the 
subject (Botanische Zeitung, 1857, 
column 116). 
In 1907 Dr. F. C. von Faber described a bacterial scab of beets which begins in the 
lenticels. This was common in Germany in 1906. 
Bacteria which have multiplied enormously in the interior of shoots may again reach 
the surface of the plants through lenticels as in case of the mulberry blight (fig. 3). 
Extrafloral nectaries——The stigma. No bacterial diseases are yet known in which 
infection takes place through extrafloral nectaries or through the stigma, but these are also 
unprotected places and such channels of infection are likely to be discovered if searched for. 
Fig. 17.* 
PERIOD OF INCUBATION. 
By this we mean the time between exposure to the cause of the disease and the first 
appearance of physical signs of disease. In plants the period of incubation is quite as 
variable as in animals. It depends on many factors, e. g., the nature of the organism, its 
food requirements, its susceptibility to plant acids and the ease with which it produces 
ammonia or trimethylamin to neutralize these acids, its temperature requirements, the 
age of the cultures used, the volume of infectious material, the age of the plant, the rapidity 
of its growth, the juiciness of the parts, and, finally, individual or varietal resistance due 
to various unknown causes. 
*Fic. 17.—Cross-section of a leaf of broom-corn, showing a later stage of stomatal infection than fig. 16, but leaf 
not yet collapsed and endodermal cells ee not yet shriveled. Bacteria fill substomatic chamber and lie over and between 
cells but not inside of any. Tissués shrunken somewhat by strong alcohol. Two stomata ss through which bacteria 
entered, as indicated by sections to either side in the series. Xylem and phloem free from infection. In upper part of 
section bacteria lie over (on) three cells, sharply delimited on one side by cell f, and on other side by cells ee. Ata 
deeper focus these three cells are free from bacteria, except for a few lying between cell-walls. Similar collections of 
bacteria along cell-walls may be seen in extreme upper part of picture. Contents of epidermis cells omitted. Slide 
412 Ag, upper row, sixth section from left. Paraffin embedded section, stained with carbol fuchsin. Drawn with a 
Zeiss 3 mm. 1.40n. a. oil immersion objective, No. 12 compensation ocular and Abbe camera, 
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