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, 7. ¢., 
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. 
