46 BACTERIAL DISEASES OF PLANTS 



The active parasites produce toxins freely, poisoning the 

 tissues, and enzymes converting starches into sugars, com- 

 plex sugars into simpler ones, and so on, for their nutrition. 

 They also neutralize and consume plant acids, and feed upon 

 amino bodies and other nitrogenous elements of the host. As 

 a result of their growth, many of them liberate both acids and 

 alkalis, to the detriment of the plant. The solvent action of 

 their products on the pectin compounds of the middle lamellae 

 separates cells and leads to the production of cavities in the 

 cortex, pith, phloem and xylem. There is also, or may be, a 

 mechanical splitting, tearing or crushing due to the enormous 

 multiplication of the bacteria within confined spaces. The 

 whole intercellular mechanism of soft plants may be honey- 

 combed and flooded in this way, and if the cavities are near the 

 surface the tissues may be lifted up or the bacteria may be forced 

 to the surface through lenticels or stomata in the form of tiny 

 beads or threads (pear, plum, bean, maize, sugar-cane, cotton, 

 mulberry, etc.), or by a splitting process. The splitting in 

 plum fruits and peach fruits, due to the black spot, results, 

 however, from local death of the attacked tissue with continued 

 growth of the surrounding uninjured parts. I now doubt if 

 any of these plant parasites consume true cellulose. 



A majority of the forms known to cause plant diseases are 

 extra-cellular parasites occupying chiefly the vessels and inter- 

 cellular spaces, causing vascular diseases, soft rots, spot dis- 

 eases, etc.; but intra-cellular parasites also occur, e.g., Bac- 

 terium leguminosarum^ causing root-nodules on legumes, and 

 Bacterium tumefaciens causing crown gall. The former multi- 

 phes within the cell myriadfold, prevents its division, destroys 

 its contents including the nucleus, and enormously stretches 

 the cell wall so that the cell becomes much larger than its normal 

 fellow cells and is packed full of the bacteria. The latter 

 does not multiply abundantly in the cell, does not enlarge it 

 greatly, does not injure its viability, and would be a harmless 

 messmate were it not for the fact that it exerts a stimulating 

 effect on the nucleus, compelling the cell to divide again and again. 



'Thi.s is a polar flagellate organism — usually it is 1-3 flagellate.' 



