Bacteria in their Relation to Vegetable Tissue. 9 



Now let us consider the two possible theories, besides that of actual 

 growth, which suggest themselves. 



First, that of diffusion. For this a fluid substance is necessary 

 that is continuous throughout the plant. As the cellulose wall and 

 ectoplasm of the vegetable cell act as an effectual filter of solid 

 particles, there would be no chance for direct diffusion from cell 

 to cell. The inability to utilize the intercellular spaces for this 

 purpose is equally evident, for these are, under normal conditions, 

 filled only with a saturated vapor, and not fluid substances, and there- 

 fore unable to function as a means of diffusion. 



If simple diffusion were operative, then, too, we would expect to 

 find the bacteria diffused as far below the point of introduction as 

 above, especially as gravity would aid in this result. This is con- 

 trary to the experimental facts. 



Now, is it possible to explain this distribution as a result of the 

 transpiration currents in the stem? 



Whatever may be the ultimate outcome of the conflicting theories 

 regarding the locus of the transpiration stream, it rests upon the 

 imbibitory and osmotic powers of certain vegetable cells. But this 

 stream can only carry substances in solution through these vegeta- 

 ble membranes, and therefore could not function as a transporter of 

 solid bodies like bacteria. This was demonstrated by cutting the 

 stem of a thrifty growing plant under water, and then transferring it to 

 a vessel containing a nutrient solution to which a dilute culture of a 

 germ had been added. It was found that in the exercise of the ordi- 

 nary processes of vegetation, no germs were detected in the tissue to 

 any considerable distance above the water level. 1 



Then, too, the distribution of the bacteria in different tissues, such 

 as the cortical and pith parenchyma, as well as the fibro-vascular 

 tissue, could hardly be explained by the action of this current. 



If the germs were mechanically transported, why was it that only 

 certain forms, notably saprophytes, were selected ? This cannot be 

 accounted for on the ground of size or independent motility of the 

 organisms. Some forms, such as B. amylovorus, were able to exist in 

 large numbers at inoculation-point for a considerable length of time, 



1 It is possible, however, that a slight amount of fluid may be, drawn up a short 

 distance into the intercellular spaces and vascular lumina, in order to equalize the 

 negative pressure of the contained air which is found often in these cavities. 



