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. In case 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 {loc. 

 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. 



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 peiiod 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 susceptibiUty 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. 



Fig. 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. Tissues shrunken somewhat by strong alcohol. Two stomata m 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 /, and on other side by cells ee At a 

 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 A9, upper row, sixth section from left. Paraffin embedded section, stained with carbol fuchsin. Drawn with a 

 Zeiss 3 mm. i .40 n. a. oil immersion objective, No. 1 2 compensation ocular and Abbe camera. 



