BACTERIA ON THE SURFACE OF PLANTS. 29 



occurs often also in leaf-spot diseases. It is not always a perfect protection, however, 

 since in some weak portion the parasite may break through the barrier and form a new 

 center of infection. This I have observed many times. 



The chemical obstacles are equally interesting, although our knowledge of many of 

 them is far from exact. There can be no doubt, however, as to their existence. There 

 are probably few substances in plants which bacteria can not be educated to tolerate in 

 test-tube cultures, especially when the inoculations are very copious and the doses of the 

 antiseptic are small at first. The conditions in nature, however, are somewhat different. 

 Especially are the number of bacteria accidentally introduced into the plant undoubtedly, 

 in most cases, vastly fewer than we introduce on our needles or with the hypodermic 

 syringe. 



It is a common laboratory experience that culture-media which will not cloud when 

 inoculated with minute doses of bacteria will do so when inoculated with larger quantities 

 of the same bacteria. 



I found the acid parenchyma juice of cucumbers exerted a decidedly retarding influence 

 on Bacillus tracheiphilus and the same was true of hyacinth juice on Bad. hyacinthi; with 

 Bad. stewarti I could not get any growth in a very acid tomato-juice. In one instance a 

 slight reduction in the acidity of a potato juice by the use of sodium hydrate enabled an 

 organism to grow readily a very slight reduction in proportion to the total acid present. 

 Many bacteria are quite sensitive to the organic acids occurring in plants, e. g., malic acid, 

 citric acid, tartaric acid, and there can be little doubt that these exert a protective influ- 

 ence. I have never found any bacterium that would grow in pieplant juice* or in orange 

 juice. Galippe states that he could not obtain any growth in garlic juice. I made one 

 trial with the same result. 



Whole families of plants contain bitter or aromatic substances, and some of these 

 must undoubtedly be regarded as protective substances, indeed, some of them as salicin, 

 methyl salicylate, thymol, menthol, camphor, cinnamon oil, mustard oil, are distinctly 

 antiseptic. Other plants contain alkaloids, glucosides, etc., which may be assumed to be 

 more or less protective. 



Tannin is a substance very widely distributed in plants ; we do not know its functions 

 very well, but, as Tschirch has suggested with reference to germinating seeds, one of them 

 may be that of antisepsis. It might inhibit bacterial development either directly or by 

 oxidation into more active colorless or brown compounds. In this connection see inter- 

 esting observations by Hiltner on a germicidal or inhibiting substance extruded by sprout- 

 ing seeds of legumes (p. 124). Anthocyan is thought by some to be antiseptic. 



Appel has pointed out that potato-tubers, the flesh of which turns a reddish brown 

 on cutting (oxidation of tannin compounds) , are much more resistant to Bacillus phytoph- 

 thorus than those which remain white. The latter were rotted easily on inoculation, 

 the former either not at all, or very slowly. In an experiment by the writer made in test- 

 tubes using this same organism, growth was certainly more rapid in potato-juice steamed 

 at once and consequently remaining pale, than in a portion of the same juice that was 

 allowed to oxidize 24 hours before steaming. Dr. O. Iyoew states that he found bacteria 

 absent or rare in brown, curing tobacco, and the writer confirmed microscopically some 

 of his results. In the samples shown to me bacteria were certainly not abundant. 



The list of weak to moderately strong antiseptics derived from plants is a long one, 

 as every bacteriologist knows. We are accustomed to look on them, for the most part, 

 with little favor because they do not accomplish all we desire, i. <?., they are not actively 

 germicidal, but the requirements of the plants are undoubtedly less, by far, than our own, 

 and in a particular case the antiseptic substance may be all sufficient for the plant, i. e., 



*In August 1908, what appeared to be a bacterial soft rot of pieplant (base of the petiole) was received from 

 Nashville, Term. A fungus, somewhat resembling a Pythium, was associated with the bacteria. 



