558 BACTERIAL DISEASES OF PLANTS 



numerous. Sometimes after a few days the whole thickness 

 of the leaf is involved, at other times the tumor is confined 

 chiefly or exclusively to the loose mesophyll, the palisade tissue 

 remaining normal or nearly normal. Always there is a striking 

 contrast between the surrounding normal, spongy, chlorophyll- 

 bearing mesophyll tissue, in which the intercellular spaces occupy 

 roughly about one-third to one-fourth of the whole leaf-space, 

 and the tumor, in which the cells are free from chloroplasts and 

 very compactly arranged, much as in embryonic tissues. Rudi- 

 mentary vascular bundles are developed in these tumors but 

 their development does not proceed very far because after 

 about two weeks the tumors begin to lose water rapidly and 

 shrivel. 



Acetic acid, formic acid, ammonia and aldehyd are all 

 capable of causing tumor development and are the substances I 

 have experimented with most because these are said to be prod- 

 ucts of the crown-gall organism and are probably common 

 products of a variety of tumor-producing parasites, and for this 

 reason are much more interesting than non-vegetable and non- 

 animal products, like copper sulphate, mercuric chlorid, etc., 

 with many of which small overgrowths may be produced, 

 probably in the same way, that is by partial paralysis of the 

 hyaloplasm and nucleus of the cell, leading to loss of water, 

 concentration of cell-sap, increased acidity, oxygen-hunger, 

 nuclear division and return movement of water and food-stuffs, 

 the stimulation resulting in a hyperplasia or a hypertrophy 

 according to the extent of the paralysis of the protoplasm. 



Let us now turn to higher types of tumors, those which are 

 complex in structure and have a self-centered growth. In 

 crown gall, as in olive tubercle and sugar-beet tubercle, large 

 overgrowths are produced by the bacteria. It will be sufficient 

 here to consider the first named tumor, since it is more highly 

 developed and more like animal cancer than either of the others. 



What then is the intimate nature of the shock leading to 

 tumor formation in crown gall, or in other words, what is its 

 secondary etiology? It is, I believe, first of all, following growth 

 of the bacteria and the extrusion of their by-products into the 

 tissues, an increased acidity in the parasitized cells and prob- 



