30 BACTERIA IN RELATION TO PLANT DISEASES. 



enough to stop the beginnings of mischief, or to delay growth until cork-barriers can be 

 formed. 



There may be also unknown substances in the plant, enzymic or other, having a special 

 protective function. If there are not some such substances, especially in the roots, it is 

 difficult to understand how plants live at all, since the roots are broken and wounded by 

 many sorts of animals, and grow in a substratum swarming with micro-organisms. The 

 writer's thought comes back frequently to the question: What protects the roots? And 

 in the case of water-plants, we may add, the stems also? for their surface is constantly 

 bathed by water containing innumerable bacteria. Wiesner has classed all plants into 

 ombrophobic and ombrophylic: the one sort having many devices for keeping out excessive 

 moisture, and rotting readily when wetted unduly; while the other sort wets readily and 

 resists decay. He found roots of all plants extraordinarily resistant to decay. Probably 

 the spongy nature of many roots and of stems of aquatics saves them from decay, i. e. , 

 they are too dry for the bacteria to obtain a foothold. 



THE EPIPHYTIC SPECIES. 



This brings us naturally to the question of what bacterial organisms are likely to be 

 found on the surface of plants. Animal pathologists know that the skin harbors a variety 

 of bacteria, and surgeons have devised a very elaborate technique for surface sterilization. 



The surface of a plant, while not excreting fermentable substances to the extent of 

 the animal skin, has, to some extent, its own peculiar flora, and a similar technique is 

 necessary if one wishes to make sterile wounds. 



The swarming bacterial flora of the surface layers of the earth (the soil proper), the 

 multitude of organisms known to occur in all waters that have touched any fertile portion 

 of the earth's surface, and the incalculably great number contained in the animal waste 

 used in agriculture, expose the surface of the plant, particularly in agricultural fields and 

 hothouses, to all sorts of contaminations. Sporiferous and non-sporiferous forms occurring 

 naturally in the soil, in water, or in dung are very likely, therefore, to be found on the surface 

 of plants and sometimes to such an extent on vegetables, fruits, and salads as to make 

 their consumption in a raw state the beginning of various intestinal disturbances. 



Some of the dung-bacteria found on plants are green fluorescent on culture media 

 and grow so rapidly that they swamp all slow-growing forms. These break up nitrogen 

 compounds into simpler substances, often into free nitrogen. One meets them very 

 frequently on the surface of plants grown in hothouses or in heavily manured fields. 



The accidental surface organisms are not the most interesting, however, nor apparently 

 are they the most frequent. There is a great deal yet to be learned about the subject, 

 but we know enough already to assert that the surface of plants harbors habitual residents, 

 bacterial epiphytes, so to speak. Those with very generalized or very limited needs are 

 found on a great variety of plants, while others seem to be much more restricted even if 

 they are not confined to particular plants or groups of plants. The subject is perhaps 

 one of no great practical interest, but none the less very interesting from the standpoint 

 of pure science. 



Many so-called soil-bacteria, water-bacteria, and sewage-bacteria are not such per se, 

 these particular organisms having their true home on the surface of plants. Bacillus 

 cloacae was described from sewage, but its true habitat is undoubtedly the surface or 

 decaying parts of plants. Bacillus coli was described from the intestinal tract where it 

 is usually, if not always, very abundant, but according to Metcalf, Prescott, and others, 

 it occurs on the surface of grain much as though at home there. Barber's temperature 

 curve for this organism makes me think, however, that it is quite as well adapted to the 

 animal body. Recently John R. Johnston, working in my laboratory, has found it to be 

 the cause of the bud-rot of the coconut-palm. 



