88 



BACTERIA IN RELATION TO PLANT DISEASES. 



the restriction of an organism to a single host-plant may be only an inference based 

 on insufficient observation rather than an actual fact. After a time the apple and 

 quince were added to the pear as host-plants, and now we may add also the plum 

 and the loquat. 



PATHOGENIC OR NON-PATHOGENIC TO ANIMALS? 



If the organism will not grow in the thermostat at 37 C, or grows only 

 feebly, as is the case with many plant parasites, it may be assumed to be non-patho- 

 genic to animals with warm blood. Only those organisms which grow readily in 

 the thermostat at 37 C., and which closely resemble animal-pathogenic forms or 

 which are suspected of causing some particular disease of animals, need be tested 



Fig. 72* 



by animal experimentation for economic purposes. In general, it is best to leave 

 this part of the work to the animal pathologist, for the same reason that the more 

 abstruse chemical problems are turned over to the chemist. 



All of the plant-parasitic bacteria, so far as tested, have turned out to be non- 

 pathogenic to warm-blooded animals, but it is not unlikely that some exceptions 

 may be discovered. 



Another question, of special interest to animal pathologists, arises here, namely, 

 whether forms known to be pathogenic to animals and especially to man are ever 



*Fic. 72. Bacterium pruni. Vertical section through a green plum fruit (var. Hale) showing 

 bacterial cavities and the escape of the organisms through the ruptured stoma. In this case beyond 

 doubt the central stoma is the one through which the infection originally took place. Drawn from 

 a photomicrograph. The material was fixed in alcohol, infiltrated with paraffin, cut on the micro- 

 tome, and differentially stained. 



