I'RKSIDKNTIAL ADDRESS SECTION C. Si 



apparently arrested by their own decomposition products. The 

 question of the production of antibodies by the plant is one which 

 has received very little attention from phytopathologists, prob- 

 ably owing; to the fact that, on account of the small individual 

 value of each plant, the prevention of disease by inoculation 

 would not be practicable, even if it were proved possible; also, in 

 very many of these diseases, given suitable atmospheric condi- 

 tions and young rapidly-growing tissues, it is possible to pro- 

 duce any number of secondary infections in close proximity to 

 bacterial lesions. 



The vascular diseases are characterised by a marked occlu- 

 sion of the vessels, which may be plugged with bacteria for a 

 considerable length. From the vessels the bacteria invade the 

 parenchyma cells in close proximity to the fibro-vascular bundles, 

 but, as a rule, only after the vessels have been broken down and 

 destroyed. Some of the organisms causing vascular diseases 

 are wound parasites, but others find their way into the tissues 

 through natural openings, e.g., B. campestre enters the water 

 pores at the tops of the leaves, and from the parenchymatous 

 tissues which it first invades, makes its way into the vascular 

 system of the plant, and for some time it is confined pretty 

 closely to the spiral and deticulated vessels. 



The best known example of the third class of bacterial 

 diseases, i.e., those causing distinct hyperplasia, is the crown gall 

 of plants caused by Bactermm tunvefaciens. Here the organism 

 is intracellular and does not multiply to any great extent, but 

 it exerts a stimulating influence on the cell nucleus which causes 

 it to divide repeatedly, with the result that large galls or tumours 

 are produced. In these overgrowth there are a number of hyper- 

 trophied cells containing several nuclei, usually not more than 

 2-4 in each cell. From the studies of Dr. Erwin F. Smith, it 

 would appear that most of the nuclear divisions accompanied by 

 cell division are by mitosis, but frequently there have been found 

 luiclei which are lobed and in process of amitotic division, and 

 it is probably in this way that the cells become multinucleate. 



Examples of hypertrophy are not only found in galls and 

 tumours, but have also been observed in other types of plant 

 disease. 



Citrus leaves attacked by the canker organism show a dis- 

 tinct hypertrophy of the cells of the mesophyll at an early stage 

 of infection, but it is not evident whether this is due directly to 

 the action of the organism, or is a result of the rupture of the 

 epidermis. Kiister (32) illustrates hypertrophied mesophyll cells 

 enlarged into vestcular swellings, on the wounded border of a 

 cabbage leaf which strongly resemble those found in an incipient 

 citrus canker, but are considerably larger. 



Tyloses are frequently found in the vessels of mulberry 

 twigs attacked by Bacterium mori, and have also been observed 

 in potato stems attacked by B. solanacearum. 



