42 



DISEASES OF CROPS. 



aim, because it was discovered upon Allium cepa. It 

 grows tolerably well in sterilized nutrient agar-agar 

 (Japanese isinglass), and other cultivating media used by 

 scientists. Upon the surfaces of nutrient media, B. 

 alia produces a bright-green pellicle, and causes certain 

 chemical changes to occur in albuminous substances. The 

 microbe is chromogenic ; i.e., it produces a pigment (of a 

 green colour), by the decomposition of the medium upon 

 which it lives. 



Fig. 16. — PniKEFACTivE Bactekium op Onions (Bacterium 



A. The Bacterium under the high powers of the microscope. 



B. Bacterium allii growing on nutrient agar -agar. 



It appears that the whole function of this organism 

 is to decompose or disintegrate albuminous substances, 

 giving rise to certain products ; viz., sulphuretted hydro- 

 gen gas (in small quantities), and a deliquescent alkaloid 

 of the following symbolic formula, CiqH^j N. 



Bacterium allii is possessed of great tenacity of life, 

 for after an exposure to 32° C. (dry heat) for six months 

 the organism had not lost its vitality. Cold appears to 



