38 BACTERIA IN RELATION TO PLANT DISEASES. 
tetanus. Nothing, for instance, is yet known among bacterial diseases of plants compar- 
able to the action of the tetanus poison. When large limbs of trees are destroyed, without 
the general distribution of the bacteria in these limbs, as in pear-blight, death results from 
the girdling action of the organism lower down upon the limb or trunk and is due to a 
mechanical injury exactly as if the limb were ligated or peeled. The whole field, however, 
has not been worked over. 
There are many grades of plant parasites from those which appear to require only the 
slightest foothold, even in vigorous subjects, to those able to attack only under conditions 
of depression or during that weakness of age preceding natural decay. In this particular, 
plant-diseases do not differ materially from animal-diseases. Probably malnutrition plays 
a large part in rendering plants and animals susceptible to disease, but when we come down 
to specific details and proper dietaries we are still very much in the dark, largely, it may 
be presumed, from the slowly cumulative effect of such influences and the lack of sufficient 
experimentation. Very vigorous looking plants and animals often succumb to disease. 
Yet even here appearances may be deceptive, and it is safe to say that in a few decades we 
shall know much more than we do at present about what really constitutes vigor in the 
sense of resistance to disease. We are now probably often deceived by appearances, desig- 
nating as vigorous, both plants and animals which, under adverse circumstances, would 
really have very little power of resistance. We know already that rapidly growing, 
luxuriantly green plants have frequently had too much nitrogen and are in a worse con- 
dition, i. e., less able to resist cold and certain diseases, than paler green, slower growing 
individuals. It is also believed by some that the presence in the soil of an abundance of 
lime and phosphates renders certain plants hardy. In case of plots of potatoes grown on 
the Potomac Flats in Washington, and treated heavily for two years at planting time with 
various standard fertilizers, e. g., lime, potash salts, phosphates, nitrates, etc., and sub- 
sequently inoculated in the foliage and green shoots with various bacteria, Bact. solana- 
cearum, Bacillus coli, the writer could not see that the previous treatment of the soil made 
any difference in the sensitiveness of the plants grown upon it. The subject, however, is 
one which invites experiment.* 
To be a parasite then, it is not necessary that injury to the host should come about 
in one specific way. As a thief may enter a house through the cellar or the roof and by 
way of an open door or a closed window, the essential thing being the fact of entrance and 
theft, so two parasitic organisms may attain the same end by two quite different ways. 
The organism may gain an entrance in any way it can, and may abstract its food from the 
host-plant in any way most congenial to it, either by ramifying exclusively in the inter- 
cellular spaces and middle lamellz, by growing through the cells, by sending haustoria 
into the cells or by secreting enzymes or toxines which destroy the cells, the substances of 
which are then used for its growth, whereupon fresh enzymes or toxines are secreted for 
the destruction of remoter cells to be in turn converted to the uses of the ever multiply- 
ing hostile organism. Such, at least, is my conception of a parasite and such is my use of 
the word. Those who wish may hold on to the old terminology, or any terminology they 
desire. That any bacteria causing diseases in plants are “‘streng obligate Parasiten,’”’ to 
use De Bary’s term, I have never maintained, neither do I believe that there are any such 
parasites whatsoever. We may retain the term, if we like, but probably it is only a con- 
venient expression to cover our ignorance. 
That bacteria can enter the host-plant in the absence of visible wounds is no longer a 
matter of doubt. They do not enter by the enzymic action of hyphal filaments, or germ- 
tubes, because they do not possess such organs, but they do so in an equivalent way; that 
is, they attack the plant through natural openings, destroying the nearest cells first, and 
*Recently Lyman J. Briggs, of the U. S. Department of Agriculture has shown that by withholding lime and 
potash and adding acid phosphate at the rate of 1,000 Ibs. per acre a serious disease of tobacco in Connecticut (due 
to the soil fungus, Thielavia bas‘cola) can be prevented almost entirely. 
