ENTRANCE OP BACTERIA INTO PLANTS. 37 



to disease is a work for the future, and our most brilliant pathological successes will lie in 

 this direction. It is, however, a task for generations. 



H. Marshall Ward's view, as expressed in his book on Disease in Plants, is also unten- 

 able. This is that the parasitism of bacteria in plants must be held in doubt until it has 

 been proved that they enter living cells in the same way as certain fungi, i.e., by enzymic 

 action, as De Bary proved the penetration of cell-walls by certain fungi.* 



The question is purely a verbal one, i.e., one of definitions. If a bacterium, or any 

 other organism for that matter, is to be considered a parasite only in so far as it conforms 

 to Marshall Ward's definition, then the case is adjudicated. Admitting the premises the 

 conclusion follows, and no more remains to be said. The error lies in the narrowness of 

 the original definition, which admits for a parasite only one mode of action, and which 

 would exclude tetanus or anthrax as effectually as pear-blight or olive-tubercle from any 

 list of parasitic diseases. 



What is a parasite ? The word is from two Greek words para, beside, and sitos, food, 

 and is very ancient. It means literally, we are told, one who eats with another. It came, 

 however, in course of time to have a bad meaning. It was used by the Greeks to designate 

 certain hangers-on at feasts who came uninvited, devoured the food and gave nothing in 

 return but fawning and flattery. With lapse of time the word has acquired various secondary 

 meanings. There has been considerable change of view even since 1884, when De Bary 

 discussed the subject in his classical Vergleichende Morphologie und Biologic der Pilze, 

 Mycetozoen und Bacterien. To-day, in a broad way, a parasite may be defined as an 

 organism which is nourished at the expense of another organism and which gives little or 

 nothing in return. The last part of the definition appears to be required to exclude the 

 relations of mother and child and various real or supposed cases of symbiosis, e. g., lichens, 

 root-nodules of I^eguminosae, etc. The following somewhat narrower definition excludes 

 the old Greek idea and its modern equivalents, but is equally applicable to the subject in 

 hand : A parasite is any organism which passes the whole or a part of its life cycle on or 

 within another unlike organism deriving its food therefrom and injuring it in the process. 

 A parasite might also be defined as an organism living with and taking its nourishment 

 from another organism, and by this union causing a disease, weakening, or malformation in 

 the attacked plant or animal. 



In the past there was sometimes added to the definition a statement that the parasite 

 was unable to obtain its food from nonliving substrata. Experiment, however, has demon- 

 strated that some of the supposedly strict parasites, those which De Bary designates as 

 obligate parasites, are able to grow on nonliving media. In other words, one after another 

 of these organisms has been transferred to the group of facultative saprophytes, while many 

 supposedly pure saprophytes have been found to be facultative parasites, e.g., many species 

 of Fusarium and the Bacillus coli. It is probable, therefore, that if this portion of the 

 definition is insisted upon de rigueur we shall in the end be reduced to the ridiculous situ- 

 ation of having no parasites at all, since at no distant time it is likely that ways will be 

 found of cultivating all of the so-called strict parasites on artificial media. There would 

 then certainly have to be some shifting of out-worn definitions, but the facts in the case 

 would remain the same, the effects of certain bacteria on living plant and animal tissues 

 would not then be more destructive than they are now. 



The direct injury caused by a parasite may be extremely slight, ^.e., confined to a few 

 cells, or may be so extensive .as to involve many systems of tissues. The indirect injury 

 in plants usually bears a rather close relation to the direct injury, i. e., to the extent of 

 multiplication of the organism, but in some of the animal diseases by reason of toxines it 

 is out of all proportion to the actual multiplication of the parasite, e.g., in diphtheria and 



*The actual statement is as follows: "but it is necessary to bear in mind that actual penetration of the cell-walls 

 from without must be proved as De Bary proved it for the germ-tubes of fungi, before the evidence that bacteria are 

 truly parasitic in livin'g plants can be called decisive." 



