1915] 



SMITH BACTERIAL DISEASES OF PLANTS 



383 



say it is customary, to find the vessels of the stem filled with 

 the bacteria continuously for a distance of 3-6 feet from the 

 point of infection, i. e., from the surface of the earth to the 

 top of the full-grown plant. In cucurbits attacked by Bacillus 

 tracheiphilus and in sugar-cane attacked by Bacterium vas- 

 cularum the same thing occurs, and many of the vessels are 

 filled solid with the bacterial slime to a distance of 8 or 10 feet 

 from the place of infection. In such cases infection has taken 

 place generally near the base of the plant, which continues to 



grow for some weeks or months. 



Transitions, of course, occur. Bacterium Stewarti, for ex- 

 ample, is confined much more strictly to the vascular bundles 

 of the maize stem than is Bacterium Solanacearum to those 

 of the tomato, potato, or tobacco stem, although it also is a 

 vascular parasite; that is, following infection of the vessels 



not find in the maize stems that extensive breaking 



we do not 



down of the pith and phloem into vast cavities which 



common, for examine, in tobacco and tomato stems. 



so 



WHAT GOVERNS INFECTION 



Within the plant we may suppose, from certain indications, 

 that abundant juiciness is the chief factor governing the in- 

 fection of immature tissues. To this may be added an abun- 

 dant well-adapted food supply and, in some cases, probably 



I 



the absence of inhibiting substances, which may appear later. 

 As the parts approach maturity the water content becomes 

 less. Along with this, acids, sugars, amids, proteids, etc., are 

 consumed and converted into substances less well adapted to 

 the needs of the meristem-parasites, if not wholly inimical. 

 In young shoots of potato and tomato, or of pear and apple, 

 as contrasted with old ones, or in the roots of carrots as com- 

 pared with the leaves, or in rapidly-growing cabbages, as 

 compared with slow-growing ones, we know that there is an 

 excess of water, and this alone appears to be sufficient to ex- 

 plain the difference in behavior of their respective parasites 

 in old versus young parts. When, however, we come to ripen- 

 ing fruits, such as the pear and the plum, it would seem that 

 they are still juicy enough to favor the growth of almost any 



