ENTRANCE OF BACTERIA INTO PLANTS. 39 



then, as they continue to multiply, those that are more remote. They do this in the same 

 way as certain fungi, *". c, by the disintegrating action of secreted soluble substances 

 enzymes, etc. 



The question might still be raised legitimately whether they can ever enter the plant, 

 or the animal for that matter, except when favored by some slight mechanical injury, e. g., 

 the death of a few cells by asphyxiation, or otherwise, to offer a starting pabulum. So far 

 as we know, they enter water-pores and stomata, with subsequent infection of the plant, 

 only in the presence of water, but in this respect they are not different from the fungi. 

 All we know definitely is that they enter the plant under conditions that are very common 

 in nature and that they set up grave disturbances, whereas, if they are not present, the 

 drops of water disappear from the surface of leaves and stems and the plants remain un- 

 harmed. In nectarial, stomatal, and water-pore infections, which are extremely common 

 in certain diseases, wounds in the ordinary meaning of that word are out of question. It 

 is conceivable that rain-drops or dew-drops might remain on the plant long enough to kill or 

 injure certain cells, which would then extrude fluids and furnish the slight amount of food 

 required by the bacterium to begin operations. This is not impossible and may even be con- 

 sidered as perhaps probable in certain diseases of rainy seasons, c. g., pelargonium leaf spot; 

 but it has not been established for any disease, and that it is a necessary postulate in all cases 

 seems unlikely for several reasons. In some of my spraying experiments with Bacterium 

 malvacearum on cotton, small round spots ascribed to suffocation appeared on some of the 

 leaves, but these were just the places which did not contract the angular leaf-spot. The 

 latter appeared later in other places on the leaves as a result of stomatal infections. The 

 writer has proven conclusively in at least one case, viz., the black rot of cabbage, that the 

 bulk of the infections take place through the water-pores, and that the fluid extruded 

 naturally from these substomatic chambers contains food enough to enable Bad. cam- 

 pestre to begin its growth. This is all that is required. In case also of Bacillus amylovorus, 

 causing the fire blight of pome fruits, the nectar of pear and apple flowers affords all the 

 food necessary for the beginnings of growth and of destructive action on the neighboring 

 cells. 



Wehmer asserts that the tissues of the potato-tuber are always asphyxiated before 

 bacterial invasion occurs, but he appears to have experimented only with saprophytes, 

 and has not established his contention. In Appel's experiments and also in my own the 

 soundest potato tubers in the driest air available in the laboratory have been rotted rapidly 

 by inoculating them with Bacillus phytophthorus. In the brown rot of potatoes due to Bad. 

 solanacearum it is not necessary that the tubers should be wetted or wounded to become 

 infected since this bacterium is capable of passing from the stems into the tubers by way 

 of the vascular bundles of the rhizome, coming to the surface only in late stages of the dis- 

 ease. 



The objections to bacterial parasitism in plants have been objections coming from 

 those not familiar with such phenomena, and we all know how difficult it is at first for new 

 ideas to make their way. Such things could not happen because they had not come within 

 the ken of the objector, or because the physical nature of plant-tissues offered (theoretic- 

 ally) an insuperable obstacle to their multiplication, or because plant juices were acid and 

 all known bacteria required an alkaline medium, or because if such diseases existed, one 

 would already have discovered them. All of these objections were the result of inductions 

 based on insufficient evidence. A thousand observations, let us say, confirmed them, but 

 then the thousand and first upset them completely. 



It was found that some bacteria could live in acid media, and that others could convert 

 acid substrata into alkaline by methods of their own. The chemical objection therefore 

 was removed. The physical one proved to be founded upon a misconception of the mode 

 of action of these organisms. There remained, therefore, as the sole basis for scepticism 

 the inertia of accumulated disbelief, the ingrained views of a generation, and, finally, the 



