BACTERIA ON THE SURFACE OF PLANTS. 31 



When we know better the bacterial flora of the surface of plants we shall be able to 

 classify the soil, water, and sewage organisms more satisfactorily, for undoubtedly a part of 

 these belong specifically to soil, to water, to sewage, that being their natural habitat. 



The organisms peculiar to the surface of plants must be assumed to remain dormant 

 under unfavorable conditions, often probably for weeks or months coming into activity 

 whenever dewfall or rainfall renders a little food available, which will be whenever any 

 dead tissues, or extruded soluble substances, even the least, are present, and that is always.* 

 The kind of organism, which grows in this moistened dead tissue or in extracts of it, will 

 depend, of course, on what is offered since the requirements of different groups of bacteria 

 are as various as the composition of different plants. This, at least, is what a variety of 

 observations would lead me to believe. 



It is a fact well known since Cohn's time that beans thrown into water will give almost 

 invariably a green fluorescent culture. Why? The simplest explanation is that their 

 composition favors the presence of this sort of organism on their surface. Housewives have 

 long known that canned fruits usually keep, while canned green corn almost invariably 

 spoils. Professional canners have learned the same thing, and consequently employ the 

 autoclave. They know that peaches are easily canned while green corn often spoils in 

 spite of special precautions. Why? The answer is that the surface flora of the one is 

 quite different from that of the other, corn favoring the growth of some very resistant 

 spore-bearing bacteria, while the fruit does not. The surface of potato -tubers also affords 

 a home for certain spore-bearing bacteria very resistant to heat. One was found to be so 

 common that Robert Koch called it the "potato bacillus," a name that has continued in 

 use and been extended to other similar forms. It is reasonable to suppose that they are 

 not accidentally present on the tuber but rather quite at home there from the fact that 

 they convert potato-starch very readily into soluble reducing substances which they can 

 use as food. The retting of flax is due to bacterial organisms which are probably common 

 on the surface of the plant, and perhaps peculiar to it. Almost any sample of hay cut up 

 and boiled for a few minutes in water, to destroy the nonsporiferous forms, will yield 

 Bacillus subtilis, or what passes for that. 



These are old and well-known illustrations. There are many others, less familiar, and 

 the subject is still so new that I can barely touch its surface. The writer ran into it in 

 1893 when he first attempted to make cultures of Bacillus tracheiphilusfrom cucumber-stems, 

 and like most tyros did not realize the necessity for entering the plant through a sterile 

 surface. Invariably I dragged rapid-growing surface organisms across my cut surfaces 

 and these contaminated the plates and either crowded out the slow-growing parasite or 

 were mistaken for it. I learned my lesson finally. Continually since then I have had to 

 do with surface-organisms in one way or another, mostly in the way of devices to circumvent 

 them, and often in literature I have had occasion to observe how they have led unsuspicious 

 persons astray. 



It is the rule in all diseases due to bacteria, whether of plants or animals, that the 

 parasite is followed sooner or later, often somewhat closely, by saprophytes of various 

 sorts, the kind depending, of course, on what organisms are peculiar to the surface of par- 

 ticular plants or animals. Given the universal presence of these surface organisms able, 

 in the absence of restraining influences, to use starch, sugars, acids, asparagin, etc., as food, 

 and it is easy to see how any little mass of dead or dying tissue, if sufficiently moist, might 

 be invaded and occupied by them speedily, this being their way of life. It is also easy to 

 understand, if surface sterilization has been insufficient, how one can get the wrong organism 

 using what seem to be proper methods, especially if the right thing is a rather slow grower 



*Diiggeli found certain saprophytic surface bacteria extremely abundant (usually millions to billions per gram) 

 in drops of fluid exuded from the water-pores of seedlings of Triticum spelta. The amount of solid matter available 

 for their use in this fluid he determined to be only 0.05 to o. i per cent, of which about half was ash. The most common 

 form in this fluid was Bacterium fluorescens. 



