(Edema of the Tomato. 105 



putrefactive germs which set up fermentations that affect adjoining 

 tissues and the leaf is slowly disorganized. Discoloration of the tissue 

 accompanies these morbid phenomena, and while they proceed also 

 through the verj' succulent parenchymatous tissue of the petiole and 

 down the stem, there appear elongated, depressed, blackened areas, 

 which eventually reach the vascular tissue of the stem. 



Frequently such morbid changes originate from cushions developed 

 on the stem, and there is reason to believe that sometimes quite exten- 

 ive areas of hypodermal tissue of the stem are stretched enormously 

 and induce similar destructive metabolism when the epidermis has 

 become too firm for its individual cells to participate in such change. 



When the development of the cushions is confined mainly to the 

 mes^phjdl areas of the leaf, or to the very small veinlets, or at their 

 terminations, they rapidly collapse and the morbid changes succeeding 

 in the mesophyll extend over areas of various sizes, which in the early 

 stages are yellowish in coloi*. In these secondary effects many of the 

 very small young leaves are affected at the apex, f rt qiiently the entire 

 apex being involved, and on the somewhat larger leaves it may be 

 confined chiefly to the base of the leaf, or the severity of the attack is 

 located there. The progress of this yellow color indicates failing 

 nutrition which is augmented by the interference produced from the 

 diseased areas in the stem. It terminates in the death of the tissues 

 involved, which then change to a dirty grayish brown color. The 

 veinlets involved become much darker in color, being nearly black, so 

 that viewed through a pocket lens the greyish brown patches show 

 frequently cross lines of Jalack or borders of the same color. 



Inquiry into the governing cause. Having observed the external 

 aspect and progress of development of these cushions, as well as their 

 minute anatomy and relation to the well being of the plant, the inquiry 

 into their cause presents itself. The existence of any fungus of 

 ordinary dimensions standing in causal relation was easily disposed of. 

 Their superficial resemblance to the erineum developed on the leaves of 

 various species of birch, maple, alder, etc., through the stimulus 

 induced by irritation from phytoptid mites,, suggested a similar sym- 

 biont upon the tomato plant. But careful examination proved this 

 suspicion to be groundless. 



By this process of elimination of suspected causes, the inquiry was 

 gradually removed from the region of the possible symbiosis of some 

 one o*f the more prominent and eas ly recognizable microscopic forms 

 to that of a possible relation of some form of bacteria which, by its 



14 



