394 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



[Vol. 2 



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 

 amido 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 middle lamellae separates cells and leads 

 to the production of cavities in the bark, \nth, phloem and 

 xylem. There is also, or may be, a mechanical splitting, tear- 

 ing or crushing due to the enormous multiplication of the bac- 

 teria within confined spaces. The whole intercellular mech- 

 anism may be honeycombed 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 stomata 

 in the form of tiny beads or threads (pear, plum, bean, maize, 

 sugar-cane, etc.), or by a splitting process. The split ling 

 in the black spot of plum fruits and peach fruits, however, 

 results from local death of the attacked tissue with continued 

 growth of the surrounding uninjured parts. 



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 mul- 

 tiplies 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 within the cell, does 

 not enlarge it, 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 cell nucleus, compelling the cell to 

 divide again and again. 



THE REACTION OF TIIK PLANT 



We now come to the reaction of the plant. What response 

 does it make to this rude invasion? Ten years ago we might 



