38 BACTieRIA 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 trimk 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 

 shghtest foothold, even in vigorous subjects, to those able to attack only under conditions 

 of depression or during that weakness qf 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 beUeved by some that the presence in the soil of an abundance of 

 Ume 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 fertihzers, e. g., Hme, potash salts, phosphates, nitrates, etc., and sub- 

 sequently inoculated in the foliage and green shoots with various bacteria, Bad. 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 lamellae, 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 obhgate 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 lbs. per acre a serious disease of tobacco in Connecticut (due 

 to the soil fungus, Thielavia bas'cola) can be prevented almost entirely. 



