DISEASE RESIST.\NCE IN PL.YNTS. ISo- 



to go into any detailed discussion of this phase of the subject. 

 Very little is really known about it. 



Some writers claim that in certain cases anatomical characters 

 have some influence on the degree of attack by fungus parasites. 

 For example, a thin-skinned potato may be more subject to scab 

 than a potato ha^•ing a thick skin. This point does not seem to be 

 satisfactorily proven. The researches of Marshall Ward and 

 others tend to show that immunity of the host plant to some true 

 obligate parasites is not due to any pecuHarities of anatomy, as- 

 abundance of plant hairs, thickness of cuticle, or number of stomates 

 per square inch, but has to do with the attraction or repulsion 

 offered by the cells of the plant to the feeding organs of the attacking^ 

 organism. A fungus may enter the plant but not be able to develop 

 after entrance. This attraction or repulsion offered to the feeding 

 organs of the fungus is believed to be due to certain little under- 

 stood chemical substances present in the cells of the plant. In 

 selecting disease resistant varieties, we are probably selecting^ 

 those which contain chemical substances which inhibit the growth 

 of the organism we are endeavoring to evade. 



There is need of much more investigation and study on this 

 phase of the subject. When we know more about these chemical 

 substances we can, perhaps, proceed more intelligently. The 

 recent work in breeding increased protein in corn, sugar in beets, 

 etc., shows us that there is variation in the amount of chemical 

 substance in plants, and that this variation can be directed along 

 desired lines by intelligent plant breeders. 



Artificial I:\imunity. 



If natural immunity is due to chemical substances present in the 

 cells of the plant which prohibit the growth of the disease producing 

 organism, the question arises as to the possibihty of producing an 

 artificial immunity in an individual by treatment with chemicals 

 which will combine or mix with the cell sap and act in the same 

 way as the obscure chemical substances present in naturally 

 immune individuals. Can we produce an artificial immunity in a. 

 plant by inoculating such chemical substances into the tissues or 

 inducing the plant to absorb them through its roots ? 



There have been many experiments conducted to test the effect 



