181 



destroyed and the cavities formed lead me to think that this substance 

 is secreted only in extremely small quantities. The results of growth 

 on different vegetable culture media point to the same conclusion. 

 No softening of the cell walls was observed in an}' of the following 

 sul)strata: Potato, sweet potato, sugar beet, coconut. A softening 

 of the middle lamella of carrot, turnip, and radish cylinders was noted 

 in old cultures/ 



A few observations were made on the related organisms. Potato, 

 coconut, rutabaga, yellow globe turnip, and radish cylinders were 

 not softened by P^. stewarti Ps. cfrnqyestrix softened cylinders of 

 >otato. rutabaga, and yellow globe turnip. 



The behavior of P^. cmnjyestris in the interior of various host plants, 

 in the absence of any other organism, indicates that a cytase must be 

 present, i. e. , closed cavities are formed. During the formation of these 

 cavities, which are fulh' occupied b}^ the- bacteria, the parenchyma cells 

 are first separated from each other by a multiplication of the organism 

 in the intercellular spaces, the walls of the cells are then crushed 

 together by the continued multiplication of the bacteria, and become 

 more and more indistinct, until they finally disappear altogether. 



In properh^ fixed, paraffin -embedded material, cut in serial section, 

 all stages of the solution of the cells and the formation of these bac- 

 terial cavities may be readily observed, especially in the easily sec- 

 tioned cabbage and turnip occupied by Ps. ca/npestris. The organisms 

 find their way into the parenchyma from the vessels, which are first 

 occupied in ways already described by the writer elsewhere. That the 

 destruction of the cell walls can be due to nothing but this organism, 

 in the disease under consideration, is shown clearly as follows: (1) 

 Because these are closed cavities, i. e.. not in open connection with 

 the surface of the plant, except at long distances from the place of 

 occurrence; (2) because these cavities occur as freely in the interior 

 of plants that have become diseased from the writer's pure-culture 

 inoculations as the}' do in those which have become diseased naturally 

 in the fields: (3) because the microscope shows the cavities to be filled 

 exclusively by bacteria: (4) because cultures made from the interior 

 of such inoculated and diseased plants have shown P^. campestins to 

 be the only organism present; (5) because all stages in the destruction 

 of the cells and in the formation of these cavities can be followed in 

 serial sections, so fixed and otherwise prepared that the relation of the 

 bacteria to the various parts of the host plant is the same as in the 

 living plant. 



7^-. pKciAieoU also forms cavities in the interior of the host plants. 

 Concerning Pn. stewarti 1 am in doubt. 



^ Since this was written, and too late to determine experimentally, it has occurred 

 to me, as the result of reading Potter's papers, that possibly this solvent action on 

 the middle lamella is due to the formation of acid ammonium oxalaU. It cannot be 

 due to oxalic acid as such since this has no solvent action on turnip tissues. 



